"old Dacron sails. ... it gets a huge belly,"
Personal rule of thumb, five years of any regular use and the sails are blown out and cannot be trimmed anyway. Ten years, and FOR SURE they're blown out if they've been used.
Is there any plastic coating (calendaring) left on the sails? Do they feel like, well, new sails? Or cheap hotel bedsheets? If they're soft enough and flexible enough to feel like any kind of bedsheets, they're blown out.
If there are any lofts local to you, they'll send someone out to stop by and look at your boat for FREE, because they know that if you need new sails--that's their business. Obviously they've got some incentive to say you need new sails...but most will tell you if you can get another year or two out of what you've got, or if they can recut it. Always worth a call.