You can, but it's kind of a pain. It does work, it's just awfully fiddly.
Go down the scarf to where the last loose stitch is. Pick at it with the tip of a needle or crochet hook and tug the yarn toward the cast-on edge of the scarf (you'll have to try it to see what I mean.) The yarn will slide back through the stitch until it's as tight as you want it. Stitch by stitch, work your way back. It's easier (still not easy, but easier) if you slide a needle through the stitches as if you were going to knit them, so you can tighten them all the same amount. Most of the time, I only do this if there's been a big snag that pulled up a bunch of stitches, where the pulled yarn needs redistributed, or if I'm almost done and the yarn is running dangerously low so that I'm willing to make the last row or two a little tighter.
Suzeeq's idea of taking off the too-big end and adding the yarn to the other end is your best bet. There won't be a seam or anything at that point. If you pick out the stitches and try to pick up from there, you'll be a half-stitch off. That isn't impossible to hide with a band of some fancy stitch that looks like a design feature, but by doing it her way you can eliminate all of the problems.