What ever hurts less.

I think its more a personal preference whether to change it from the top or bottom (how the jewelry is designed also dictates it). If its the first time you've done it, meaning you're swapping out the initial barbell, the bottom will probably be the one that unscrews. I find it easier to find the hole on the top of my tongue rather than the bottom, but that's just me. I was told that if it comes unscrewed for any reason, if the ball is unscrewed underneath your tongue it is harder to swallow as opposed to if it came unscrewed on top. As for how long to wait, they use a longer barbell when they initially pierce it, and that can make it easier to bite down on it when your tongue isn't swelling up anymore. I found that after two weeks mine was healed enough to put a shorter barbell in, just so it was easier to eat and talk without fear of biting on it because you don't have to deal with the extra length. I think its originally pierced with a 3/4" long one and people then decrease the barbell length to either 5/8" or 1/2" depending on the person. It was a little weird changing it for the first time, but I think it is better to change it to a shorter one after 2 weeks just to talk and eat easier. I'm not advocating that you change it whenever you want after two weeks (and NOT AT ALL advocating a vibrating one, wait a while for that) just a shorter one to allow you to cope with it while it heals. Leave that one in for a few more weeks before you change it at will.