According to AT&T's Seth Bloom, you will be able to call in or hop online and change that month to the 2GB plan so long as you do so before the end of that billing cycle. So if you have a bad month (or a really good month -- it really depends on how you look at data flexibility), AT&T will offer more overage flexibility. If you do not do this, you will be charged $15 for each 200MB you use -- and that can quickly add up to a lot of ouch. As Bloom added, "You can keep going back and forth between the two tiers." So if you opt into the 2GB plan for a month, you can opt right back out the next month without penalty.

Can you add tethering for just a month? Say when we travel? Tethering does not require a contract, so you should be able to add the feature and remove it as needed for as many months as required.

Looks like my two main questions have been "officially" answered by AT&T. You can change data plans monthly, and you can add tethering for a month at a time.