Lucario At Service
Calm Trainer
Paul also had the distinct advantage of knowing what Ash had ahead of time and how to prepare which was another thing Reggie pointed out.
It's true his sinnoh pokemon have come very far in the short amount of time since then, but even so Ash does need to use the suprise factor against Paul as well and throw him off his game(after all Paul doesn't know Ash has all these pokemon.), if he goes in with the same pokemon he used before, even though he has a new one and two evolved, Paul still has the edge granted Infernape and Staraptor would take some pokemon down with them before they fell...but Paul would still win because the outcome would be the same, if Ash wants to mess up Paul's game he's gotta use pokemon Paul won't see coming.
You are right. Ash has to do something different from how he did in the full-battle because Paul had already predicted how the match would go on (if i am right, he even must have predicted that Ash would continue using Grotle against Houchkrow when he switched it in place of Electabuzz, the same goes for Buizel's case as well).
Since Paul has read Ash's battling methods completely, Ash must also respond to that in same way as well by changing his battle style.
Even though this would be a off-topic, but based on my assumption only Pikachu, Infernape & Torterra from the present Sinnoh team are the right choice for using against Paul (but he has to look out for not doing the same mistakes he did in that full battle, when he used those 3). The rest three have to be some other pokemons.
As for Paul's team, i can only say that he would use Torterra & Electabuzz/Electivire for the match. The other are unknown facts.