There are a LOT of 26 year-old boats that have NO need to replace chain plates -- however, there are some boats that either have bad details, defective materials OR have been sailed very hard and/or very long and might have some issues.

If you're going to go to the touble of opening up the hull to really test them -- then I'd replace them.

What makes you think that there is some problem with them?

If you won't be able to take the boat out and sail it (in the kind of seaway that you're going in) without worrying constantly about it, then replace that -- along with all of the standing rigging.

Where are you planning on sailing, what time of the year(s), etc.?