I'm going to have to disagree with Tom. I just bought a Genesis 330 and while assembling it myself, made the mistake of putting a locking caster in the back position, rather than the front. It seems impossible to remove the casters without also damaging the plastic housing they plug into. There's a brass collar around the caster shaft that locks them in place so they don't fall out, a very sensible precaution I wish other manufacturers would adopt.
If you're able to pull the casters out of the housing, you're a better man than I. But what you will probably have more success with is what I had to do - I just pulled the square plastic housing out of the bottom of the grill and swapped position. If you can't manage to get the caster out of the insert, just order new plastic housings and start from scratch. I'm guessing they shouldn't be too expensive.
Good luck!