If mufasa’s suggestion works for you, that’s best. Otherwise, if the BPM are indeed half, you can double them by clicking the x2 on the grid editor. But, you’d have to do that one-by-one. If you studied up maybe you knew that.
Manually set the BPM in the grid editor. If you don’t know it set the beat grid on the first downbeat and iterate until the grid matches.
Import the collection from Serato. I sometimes do that with Rekordbox and it imports the beat grid. It even imports Rekordbox dynamic beat grids. In the latter, I prefer to warp classic disco, funk and rock with Ableton (or buy a modern edit).