EV's can be removed with berries in Gen V, you just need I believe maybe 20+ EV Reducing Berries to wipe out all the EV's in a specific stat (stats being HP, Attack, Defense, Sp. Atk, Sp. Def, and Speed). Like say I want to get rid of 100 EV's in Speed, I just feed 20+ Tamato Berries to that Pokemon and it's EV's in Speed will go down to 0 so you can re-EV train it in Speed again. If your Pokemon in Gen V doesn't have 510 EV's at Lv. 100 you can still EV train it to get there depending on what Pokemon you faint and what EV's it gives you. You can also use the Xtransceiver in BW2 to contact Bianca and she will tell you If any of your Pokemon have maxed out their EV's in specific stats.
The only way EV's can't be removed for Pokemon in Gen VI for XY would be If you aren't able to import Pokemon over from Gen V to Gen VI or that Game Freak decides to remove EV Reducing Berries from XY like they did in Gen V when they were exclusive to the Dream World before becoming more accessible in BW2. The new Pokemon Amie feature doesn't allow you to feed berries or poffins from Gen IV to Pokemon in Gen VI, which I thought was strange but maybe it suggests that Gen VI has something to replace berries and poffins that you feed to your Pokemon. As for Kalos region Pokemon, I'm sure Game Freak will find some way for you to remove EV's from them but yeah as for the old Pokemon If you're allowed to transfer them to Gen VI it shouldn't be a problem.