Oblivion also had its initial T rating changed to M after I believe some cave was discovered that was considered too violent. Or it could have been a nude mod. Either way I remember it being kinda controversial of a change.
EDIT:
The pertinent content causing the change in the ESRB rating involves more detailed depictions of blood and gore than were considered in the original rating of the game (the game already carried a Blood and Gore content descriptor), as well as the presence in the PC version of the game of a locked-out art file that, if accessed by using an apparently unauthorized third party tool, allows the user to play the game with topless versions of female characters. The locked-out topless skin was found by ESRB to exist in a fully rendered form on the game disc, but is not accessible in the Xbox 360™ version of the game.