Normally you want to do it in fall through winter, when they are dormant. You can do it now, but you'll have to be careful about watering when you move them. For mine, the tips are just coming up. So if they are like that, you can probably still move them without harming them. But hostas are hard to kill, and they multiply. so if you lose a few, it's probably not a bad thing.

try to move the ones that are just emerging, divide them if you want if looks to be a large clump. one good soaking after you move them should be enough.

I have moved hostas year-round with no permanent bad effects. If you move them after the leaves unfurl and it's getting hot, you're going to have sad-looking hostas for the rest of this season, and they'll probably go dormant a little early. But then next year they'll be back more beautiful than ever. I've even divided them in full leaf, with the same results. Hostas are hard to really kill- they just might need a year or two to recover. Just DON'T transplant them to a sunny spot- they burn easily and won't forgive you.