Correct me if I am wrong but doesnít your boat have three chainplates on each side? If thatís true and the mast is on the keel you will not have any problem reworking one chainplate on each side at the same time.
I doubt your chainplates are just glassed in. Under the glass I am sure you will find bolts. Some small boats have plates glassed in but all the larger boats I know of are bolted.
Good luck and all the best,