If I change my plan in the middle of a billing period, does it take effect immediately or does it start at the beginning of the next billing period?
I ask this mostly because my self serve account says I still have the 200mb data limit, and my new plan is supposed to have 500mb
If you changed it in Self Serve, it's effective immediately, which causes proration, and is explained on the confirmation page. If you did it over the phone, they should ask if you wanted it immediately or effective as of your next billing cycle to avoid proration on your next bill.
Sometimes Self Serve will still show your old plan details for a while. There's a delay. Either wait a few hours and log in again or try clearing your cache/cookies and log in again.