Well the S III uses micro SIM so you'll have to either cut your old one or the rep will activate the micro for you, which is included for free if you're activating a phone. It supports microSD so you're safe there.
You'll have to pay off your existing negative Tab (if any) to upgrade. This can be done in Self Serve or placed on your next bill when you go in to upgrade.
You can keep your old plan if you take the S III on Tab small (150), which means you'll have to pay $150 in store for the device. If you want it for $0 in store and get a -300 Tab (medium), then you'll have to switch to at least a $30 plan + $5 tab charge until it's paid off. I don't recommend this for you, since you have an expired $20 plan. Enjoy it!