Of course, if someone would convince him today to drastically change, he might just do so, but...If Sean Kelly can win a GT, so can Sagan....He'll need a bit of luck on his side, of course...
Can't see this happening, he would really need to drastically change his weight/training schedule and would need to forego focusing on the classics
(which, without currently actually having won one of the big ones yet, won't happen at all I believe)
(and let's not forget he does win a lot of minor stages/races with his current build as well)
By the time he would feel satisfied with his victory total using his current build (especially in the classics), it would be too late to make a real contender out of him