How and if to add a 1 1/2 fret
Instruments- discuss specific features, luthiers, instrument problems & questions
That's the $100,000 question, George. Fine dulcimers made by known builders have recognized value in the marketplace, but sometimes they're not set-up the way we would prefer them to be set-up. So the question becomes "Do I change things and ruin the recognized value (but be able to play them as I wish), or do I leave them as-is, and not want to play them?" You indeed may want to (or need to) sell them at some future date.
Years ago I found a Warren A. May poplar dulcimer on eBay, selling for an attractive price. I won the auction, and when the dulcimer arrived, I found that it sounded odd to me at the low end. The first fret sounded way off, so I considered changing its position. Then I ran into the same quandary that you find yourself in now. Incidentally, I inadvertently ruined that dulcimer's fine lacquer finish by taking the very cold dulcimer out of the box in a warm room. The finish crazed instantly! The good news is that I resold it on eBay (for a higher price!) to a person who didn't mind the crazed finish.