Thread:Wingstrike/@comment-31088226-20171003212653/@comment-3542404-20171004175024

Probably during Gryps (UC 87 to 88), seeing how the Titans (later the New Desides ): "turned rogue, allying themselves with Axis Zeon"

- Titans page on Gundam Wiki

While I can't say I know of many turncoats within the UC timeline, most that I could find seemed to be from Zeon to the EFF, and those that turned from the EFF to Zeon were deep cover spies (and Nina Purpleton). So there aren't many examples of traitors within the EFF to work from. It's also somewhat tricky to justify, I can't imagine many people from Earth would sympathise with Zeon following UC 83 after the colony drop.

I suggest getting Mikel's opinion on this as well, but your best odds are that most people would have changed sides during Gryps after seeing the Titans do it. While they were allegedly destroyed by the colony laser, enough survived to found the New Desides. Additionally some EFF personnel could have seen the Titan's actions and been so blindly obsessed that they considered the same, despite the Titan's infamous reputation. The biggest question is how many people would have jumped sides together, and at most I think you could probably say a mobile suit squad/team max. You could say more if the Titans had merged with Zeon, but I don't know if that ever happened.

In my opinion the UC is pretty fixed with who plays on which team (with some minor movement, like AEUG and Karaba personnel joining Londo Bell, which is part of the EFF), unlike say in the Cosmic Era where soldiers from ZAFT, ORB and OMNI were known to have abandoned their post joined someone else (granted, changing teams from OMNI to ZAFT was probably unheard of, which is similar to EFF to Zeon).