Roy Halladay has been one of the most dominant starting pitchers in MLB since 2002. In a career that has gotten himself 199 wins and over 2000 Ks, he also is the active leader in complete games (66) and shutouts (20). Very few pitchers are able to continue to throw hard towards the latter parts of their career. Roger Clemens is a rare exception, as hard throwers like Pedro Martinez, John Smoltz and even Randy Johnson had to figure out other ways to get a hitter out. One of Halladay's strengths has been his ability to own both sides of the plate. I think it is quite possible that he can re-invent himself. Unfortunately, it is not something that will happen right away.
Phillies fans can make an example of Jamie Moyer, who seemed to be able to get hitters out no matter how slow he threw. Greg Maddux was the master of location, which made his transition seemless. Tom Glavine saw difficulty once his fastball lost velocity, and had to learn how to throw inside more. The best example of a pitcher who had a fastball and had to learn to pitch without it is Frank Tanana, a fireball throwing left hander with the Angels, who became a soft tossing lefty for the Tigers in his later years. Even greats like Walter Johnson and Bob Feller had to learn to pitch after their fastball was no longer potent.
Halladay has the benefit of not being the sole dependent in the Phillies pitching staff. Having Cole Hamels and Cliff Lee will take a little pressure Halladay for having to be the man. What he has on his side is his ability to pitch. He just has to work on the differential between his fastball and change-up. It may take some time, but I see Halladay making the correct adjustments and re-inventing himself. This is something that will eventually happen to guys like Justin Verlander, CC Sabathia and may have already happened to Tim Lincecum. It is something that Johan Santana already had to deal with before his shoulder tore to never become the same. It is going to take a lot of work, but I see Halladay becoming an effective top of the rotation starter again. Just not the pitcher that will throw complete games on a consistent basis anymore.