Like, I'm not even talking about complete reimaginings of songs incase the purists start whimpering and whining. The Blackout medley from Piece Of Me pretty much plays all three tracks straight with slightly more muscle in the instrumentals and it makes the world of difference. Stuff like "Poker Face" is in dire need of a similarly fresh lick of paint.
Britney may rely on 1998 vocals, but I do always appreciate her keeping hits fresh sonically for the live shows. She's was bored of
...Baby, One More Time by the time the damn Dream Within a Dream Tour came around, and it's been totally remixed for every single show since (DWAD, Onyx, Circus, Femme Fatale & Piece of Me have all had different versions of the song, with Onyx probably being the most it was ever rearranged). I've seen fans complain, but I think it keeps things interesting as someone who sees her at every tour. She usually remixes 50% or more of her older hits for every show... I think there can be some compromise for Gaga.
Brit does have some tragic remix examples, though: Toxic at Femme Fatale was butchered.