I've had 14 piercings in the last year and I've found that you can change them from pretty much after the swellings gone down but it's better to leave the bar/studs in for a couple of months. Saying that, I got one a year ago and it's still healing and swells quite a lot... all my lobe ones have healed fine in a couple of days and they've been done with guns (:
Wow thats a lot. It makes my 6 ear piercings sound paltry. (3 in lleft earlobe, 1 in left helix plus 2 in right earlobe).