I didn't interview anyone or ask anyone, but I thought this was already common knowledge. Now it has become an urban legend and 'rumors.' In 1994, Austin St. John (Jason), Thuy Trang (Trini), and Walter Jones (Zack) asked for more money at the height of the Power Ranger popularity and were then fired. Power Rangers was not in the Screen Actor's Guild, a union and the three actors wanted to them to be part of the union. Saban did go to the union around Lost Galaxy or Lightspeed. It then Saban started Samurai, they are no longer with the union. Amy Jo Johnson said in an interview with No Pink Spandex, she didn't understand stuff like unions back then when she was younger, if she did, she would had supported the other three. This was discussed and confirmed in a number of interviews and articles. Then the other three replaced them. What some fans didn't notice was that the actors were not even present for the 'Power Transfer' episode and they had been absent for several episodes before that and recycled footage was used. I was 12 at the time and I noticed this. It was obvious since Zack's hair had changed from its dreads to flat hair. Also, auxiliary characters Richie (Trini's love interest) and Curtis (Zack's cousin) were not seen again either, without being 'written out' of the show.
Episode 76 - Beauty and the Beasts (10/10/1994)
Austin St. John (Jason) was not present in this episode and Jason's voice was done by an uncredited person other than Austin.
Episode 77-78 - White Light Part 1 & 2 (10/17 to 10/18/1994)
There is now a new rumor, which I am taking as a rumor, but that the Zordon line that Tommy is the new leader was a new line that Austin St. John did not know about. This would make sense because when it aired, I wondered why Jason did not react when Zordon said this. And in a later episode, Jason lead the morph instead of Tommy, leading some to believe it had not be decided Tommy was the leader when that was shot. This is when we see the new morph of Tommy in a tanktop, we don't see Billy and Kimberly's until "Goldar's Vice Versa," meaning Jason Frank probably didn't shoot this until the new three actors came in.
Episode 79 - Two For One (10/24/1994)
The three appear in this episode but Jason and Trini have very little lines in the scenes they appear in.
Episode 80 - Opposites Attract (10/25/1994)
This is the last episode the three actors appear in and is the last episode Curtis and Richie appear in as well. In this episode, Jason leads the morph; This is because Jason was originally was going to remain the leader of the team even though Tommy became the White Ranger, but the plans and references for this were cut out when St. John was no longer filming episodes on the show.
Episode 81 - Zedd's Monster Mash (10/28/1994)
This is the first episode in which the trio did not appear. No footage of them was used, it was said Trini, Zack and Jason were looking over kids that were trick or treating. This is an episode that for me established that Tommy, Kimberly and Billy had to hold the show on their own in order to distract the fact that the other three are missing.
Episode 82-84 - The Ninja Encounter Part 1-3 (11/2-4/1994)
This episode introduced Aisha, Rocky, and Adam, but the actor credits were not showed until "Goldar's Vice Versa." I think the reason the Hatchasaurus was used was because of this fiasco, I think if the trio didn't leave, this footage would have not been used. Even though the martial arts competition is not long after the picnic in the park, Trini & Jason were wearing different clothes. In the 1st part, footage of St. John, Trang, and Jones for this episode were recycled from "The Song Of Guitardo" and "Two Heads Are Better Than One". In the third part, footage was recycled from "Two Heads Are Better Than One". Also, the 'Jetting' was invented, I think to excuse why the trio were in their suits and a second excuse not to see the actors was when the trio were frozen by the cherry blossom tree monster. Also, laughable part was when Billy, Kim, and Tommy go to the competition, they say hi to stand-ins for the original 3 and they went to sit in another area. They always usually sit in the same place, and no reason was given why they sit in two different places. In the end of the three-parter, Aisha, Rocky and Adam were sworn in by Zordon, becoming 'friends' that know their identities.
Stand-ins who fans still haven't been able to identify.
Episode 85 - A Monster of Global Proportions (11/5/94)
Footage of the trio for this episode is recycled from "The Wanna-Be Ranger". The Power Cannon was not introduced yet but the monster from Dairanger, the cannon was used. The Power Blaster was used instead but we see the effect of the Power Cannon. The Peace Conference is established in this episode as well.
Episode 86 - Zedd Waves (11/7/94)
The trio were seen in their suits and manipulated by the waves. Aisha, Rocky and Adam help once again and I guess the reason there were two episodes until they got the powers was to ease fans into these characters.
Episode 87-88 - The Power Transfer Part 1 & 2 (11/8-9/1994)
Footage of St. John, Trang, and Jones for this episode was recycled from both parts of "Green No More". Tommy wears absolutely no white in the two episodes, due to the footage recycling, which makes me believe they had to match up with the footage of the missing three. If you notice the Command Center scenes, they were arranged it so the old three would appear on one side and Billy, Kimberly and Tommy in another. It is also the final credited appearance of Romy J. Sharf as Alpha 5's suit actress. The most obvious sign that Austin St. John, Thuy Trang and Walter Jones were not present were when there was an unnamed brunette playing Trini and there is a noticeable shift in her voice. They use the new lady's voice in the first half of the sentence and old Trini footage at the end and reaction shots from Zack and Jason. Reportedly, Jason's voice was done by associate producer Paul Rosenthal and Zack's voice was done by Joel Rogers, Bob Manahan's second engineer.
Episode 89 - Goldar's Vice Versa
We now see all the Rangers in their tanktops in the morph sequence, bringing on to believe Jason Frank shot his along with the new three actors OR the other five shot it in order to look like Tommy's.
I can't believe all these things happened in behind the scenes, also with the new news I got that Brad Hawkins was supposed to be the original White Ranger. So there was all these stages. Stage One: It was supposed to be Jason, Trini, Billy, Zack, Kimberly and unnamed new guy White Ranger (played by Brad Hawkins) for the whole season. Tommy was too popular so they brought him back (since he shot the Cybertron pilot) and Jason Frank and Brad Hawkins switched places. Stage Two: The original trio were dumped and once 'White Light' was already shot, Tommy was made the leader. Stage Three: Episodes were shot with used footage of the old trio and the new trio were introduced. So, there is such a thing known as 'Murphy's Law' that when things can go wrong... they do. Nothing comes out how one expects it to. Season 2 of Power Rangers sure was a doozy, with mixing footage of new monsters with another megazord and then using old footage of actors that were gone. I remember an interview with Jason Frank in TV Guide when the movie came out and he said he was happy to stay and that on the set, the crew would joke, "You better be good or we will send you to a peace conference."
UPDATED 12/21/13:Luckily, when Kimberly left it was pleasant and she got a proper farewell and the character left to train as a Gymnast, that being her dream. And Aisha had a complicated send-off, Karen Ashley wanted a proper farewell, she was promised one but there wasn't enough time to do so. And as Rocky did get a proper farewell as well, the reason was a back injury. Billy didn't get a proper farewell, two actors filled in for him--an actor playing the 'older' Billy and another in a 'garbled' message. But those four actors didn't leave because of money. Amy Jo, Karen, and Steven all left because they wanted to move on with their lives. And rumors say David Yost had creative differences with the crew. In late 2009, David Yost revealed to No Pink Spandex that he had to deal with anti-gay slurs from the crew.