There are a couple of things that have not been mentioned. One is that if you have had them serviced annually for 25 years, you have now spent far more in servicing these regs than they originally cost, and as occasional divers, probably far more than is necessary. The other is that these regulators are sold with a lifetime warranty. It sounds as if you have honored your end of the warranty requirements, and as such, if they cannot be serviced, any dealer should replace these regs free of charge. No "upgrade" or "trade in" B.S., just free replacement with whatever model is closest to what you now have. That's probably a MK25/G250V for your MK10 and a MK2+ for your MK200. If SP does not still have any G250Vs (I think that has been discontinued) and IF the problem you are having is only in your first stage, they could replace the MK10 with a MK25 1st stage and you could keep using your G250. That would be a good solution.
But most likely your regs are fine, unless you have a pitted ambient chamber. That really can be trouble, although a few of us DIYers have successfully cleaned up corroded ambient chambers with a bit of fine sandpaper/micromesh/polishing compound. A dealer won't do that. If you do have pitted ambient chambers, it's likely because you did not adequately soak your regs after saltwater use. Regardless, that's not your problem, you've already put plenty of money in SP's pocket in service costs. I suspect they will gladly replace your regs. It's a tough business and honoring your warranty will more than pay for itself in terms of good PR.
BTW, 25 years is not old for a well made regulator. None of the regs I use (and I have many to choose from) are as young as yours.