As MichaelW reported it is probably possible to run presta valves indefinitely in schrader holes with no ill effect. I tend to take the conservative route and use adapters. You have at least 3 choices.
1. Aluminum adapters
2. Rubber adapters
3. Replacement threaded collar that has a lip to fill the excess space.

I have used 1 and 3. The aluminum adapters worked great on a modern double wall rim, but were too long for an older single wall rim. The replacement threaded coller was the answer for that since the lip is only as deep as the rim wall. Your LBS should have or be able to get any of these for just a couple of bucks.