QQ: If I have an A/B test flag & I change the percentage, does that reshuffle the users, or does that new percentage affect only newly exposed users?

Users along the boundaries will switch variations. For example, if you go from a 45/55 split to a 55/45 split, most users will stay the same, but a few will switch from B to A.

thanks

If you keep the split the same, but just scale down/up traffic noone will switch variations. So going from 10/10 to 50/50 is fine