If the construction is similar to my 1985 Sabre 34; As Maine Sail says the chainplate replacement is pretty simple. However, if the chainplates are corroded and need replacing it you might have the additional problem of having to replace the bulkheads that they are attached to? ( to be determined) In which case, you would need to step the mast, remove all the wood trim and cabinetry to access the bulkheads. A proper inspection would determine if the bulkheads needed replacement. On my vessel, Sabre covered them with vinyl, so you might have to peel some of that back to expose the wood and sound it out, or use an awl or something to determine if the water intrusion caused any rot. Also these bulkheads are tabbed in to the hull. They would need to be removed and the old tabbing would need to be ground out before the new bulkheads could be made up and tabbed in. ( if you can get the old bulkheads out in one piece you can use them as templates, otherwise you could try to template them in place, before removal. If the above is the case, this may not be as simple as just replacing the chainplates, which is pretty straightforward.
I'm going to try to attach a photo of an example.
If the bulkheads have any rot, this is a much larger project than simply replacing the chainplates. Most of the bulkheads are pretty accessible. The forward shroud in the Head is the most buried. If you have any dimpling of the hull around the chainplates, it would be one indication that the bulkheads have been weakened. I don't want to scare you, but be careful and inspect everything.