I'm level 80 Prestige 15 on PS3. I'm able to go to Prestige-Mode (after the new update). What's going to happen?
I can assume it would be the same as any of the other times you prestige.
The max level simply means you reached the highest possible rank you can.
Check this link here for more information on Prestiging.
So simply: Nothing happens more or less then a normal prestige.
Infinity Ward will allow to let us Prestige until Prestige 20. But at the moment this isn't possible (maybe next Update will unlock the last 5 Prestiges).
If I Prestige NOW, I will get Prestige 15 again and start from level 1 (Can get a Prestige Token doing that) but thats everything.