As long as the salinity is the same between the two and you're shooting for 1.02375 or so (reefs should be a bit higher...like 1.025), then I think you'll be fine. I'd wait a day or two and let your new water mix, if possible.
Ordinarily, you should top off every day or two with fresh water, not saltwater, to replace the evaporated water. Top-offs with saltwater, as you probably know, will increase your salinity to dangerous levels quite quickly.