I don' t think its do to any "decline" or back problems. U2 rearrange songs all the time. "Bullet The Blue Sky" changes with every tour, especially the way Larry and Adam approach it. And usually the new arrangement tends to compliment the tour so the older songs fit in with the new. I think perhaps for the Vertigo tour, Larry sat behind the kit and thought about how he would approach the song if the band had written it in 2005. So the beat became more of a driving rock (ala HTDAAB) rhythm and less of a pulsating/pounding thing. Of course not all the older songs can be rearranged so drastically. "New Years Day" is always gonna sound like "New Years Day" etc. But they even messed with "I Will Follow" for PopMart so all bets are off as far as sacred cow songs.
But getting back to the original post, no, I don't think its a decline. Drummers tend to improve with age, especially if they play as often as Larry does. His playing has changed, his style has changed and developed but that's only natural. Most drummers start out playing too many fills and trying too hard. With time and experience, you learn more subtle ways to serve the song and usually that means doing less of the flashy stuff.
That said, just as a subjective thing... my personal favorite is the ZOO TV version. It's just more fun to play.
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