Originally Posted by Chris_W

You should have no problems replacing the large ring on a 4503 crankset. The one issue might come in the placement of the front derailleur - to get the inner plate of the derailleur to clear the middle ring, then the outer plate will be quite a bit higher than the big ring. It should still shift OK, but there will be a slightly increased chance of overshifting the chain to fall off the outside of the big ring. However, if you dial in the upper limit screw just perfectly then it should be OK. Certainly don't use an Ultegra FD (because the difference in height between the inner and outer plates is even larger); if you're using a 9-speed chain then go for the 4503 FD, if it is a 10-speed chain then use the 4603, 5603, and 5703 FDs - they should all work equally well.