You'll need to change the bushings & screws. Be VERY careful removing the bushings...it is very easy to bugger the screwdriver slot in the bushing. Also be very careful about putting the new bushings in - you don't want to strip the threads on the frame.
There is a small chance that your bushings are staked - which means you will not be able to remove them. I remember reading that one or two 1911 manufacturers stake their bushings but don't remember which ones they are. Just be careful with it and you should be ok.