UPDATED 2/8/11
I thought this was self-explanatory but this is FOR GOKAIGER ONLY! The Gokaiger become past Rangers from the past 34 teams. They borrow or copy the powers. When a female becomes a male character, she dons a skirt. When a male becomes a female character, the skirt is gone. They are NOT REPLACING the warriors, rangers or heroes. This is only for the 2011 Super Sentai series.
This whole skirt-switching thing has become a big debate among Sentai fans (even Japanese fans). I don't know about Japanese fans. I just want to say that this is no way the end (some fans thinks this is the end of Sentai) and just because Gokai Yellow dons a skirt when she becomes Big One, doesn't mean Big One himself is gaining a skirt, nor that it is permanent. It is not him, it is Gokai Yellow. Here are pictures I made for fans that may not be familiar with the suits, or the layperson who might not know how many were changed for Go-Kaiger.

Pink became Go-On Black (ゴーオンブラック), Pink became GekiChopper (ゲキチョッパー), Gokai Blue became MagiBlue (マジブルー), Yellow became Magi Yellow (マジイエロー), Hurricane Yellow (陸忍ハリケンイエロー), Gokai Blue became Hurricanger Blue (水忍ハリケンブルー ), Pink became Kuwagata Raiger (牙忍クワガライジャー), Gokai Yellow becomes GaoYellow (ガオイエロー), Time Yellow (タイムイエロー), Go Yellow (ゴーイエロー), Ginga Yellow (ギンガイエロー), Tiger Ranger ( タイガーレンジャー)
Gokai Yellow became Ninja Yellow (ニンジャイエロー ), Kirin Ranger (キリンレンジャー), Yellow Owl (イエローオウル), Gokai Blue becomes Blue Swallow (ブルースワロー), Yellow and Pink became Geki Red (ゲキレッド), and Pink and Yellow became Magi Red (マジレッド ). And in Episode 3, Yellow became Ninja Black. Gokai Pink has become Green Flash twice (in 43 and 49). Yellow became Shishi Ranger in 43. Gokai Silver has yet to become a male version of Go-On Silver. Gokai Pink became Ginga Red in 45 and Yellow became Ginga Red in 48. Gokai Blue became Pink Flash in Gokaiger vs. Gavan. Gokai Red became Pteraranger in Gokaiger vs. Gavan. Boys also became Denji Pink and Five Pink but those were gender-neutral suits. Yellow became Zuuban but with no alteration to the costume, Zuuban is pretty much genderless but has a male voice. In the Gokaiger stageshows, Yellow became Deka Red and Go-On Red too.
This is why I love Super Sentai and Toei, they are always mixing it up. They establish the rules (most girls get skirts, most girls are white, yellow, pink and sometimes blue), and then break it all down. In Gokaiger, the girls have let to be almost every color (save for Violet).
I wanted to know if the Japanese fans felt the same way and I found this:
http://logsoku.com/thread/toki.2ch.net/sfx/1295796139/
And they do say 'Why does Big One have a skirt?' and 'Miss America has a deeper Pink.' and 'Pink is not the place that you mention it Abarekiller.' Some interesting comments were: "It is a sexy Kuwagaraija." They also speculate that the Sixth Ranger might summon Rangers like Kamen Rider Diend. The grammar is off because of google translator. Someone was even asking about Mele and Rio.