If I really like the look and fit of a bra, I duplicate or triplicate. That's why I'd stitch the straps in two of the bras and reserve the third for racer backs. I've tried some of the hacks mentioned above in the past, but I've always been unhappy with the look and feel of the result. A couple of small stitches, on the other hand, is neat, pretty, and works like a charm with no malfunctions.
If you really want do something to the strap, the best solution I've ever come up with is to purchase a card of those tiny, clear plastic snaps. A couple of stitches in a matching thread will hold the both sides of the snap in place, and let you extend the length when you unsnap. The big hassle is when you need to pass one side of the snap through the strap holder because the snaps are easy to break. That's why you need a card of them, so you can replace a broken one. Rather a pain, IMHO, so that's why I prefer to duplicate and reserve one bra for racer backs.