If he cheated Taker out of the streak at Wrestlemania, he'd be booed again, for sure. Of course, that'd be a terrible booking decision, but short of that I can't imagine what else would make the entire crowd turn on him.
Watching his promo on SD I saw glimpses of the old Cerebral Assassin Triple H and I liked it. I don't think that he has to turn full blown heel. But if they decide to play up this dark side of him that he keeps talking about I think it would lead to a much more badass character, one that we haven't seen in a while. I'd mark.
I don't think they will, but they could even turn HHH on the lead up to Mania. The way Shawn Michaels has been talking about him in those promos is basically saying 'he's a heel, but he's my friend'. Like, he's heartless and cold in the ring and shows no remorse even to his friends, that's a heel if I ever saw one. I think it would take one in ring promo to get the fans to boo HHH.