There seems to be a lot of confusion of the actual "official policy". Let me clarify:
30 Day Worry Free Guarantee
This is for new AND existing customers.
You are allowed to return your device within 30 days of purchase. A restocking fee of $35 will occur.
You are allowed to exchange a device within 30 days of purchase. A restocking fee of $35 will occur. You are allowed 1 exchange within a 30 day window from your original purchase. (This means if you buy a phone and return it and wait a week, it is still considered an exchange. ONLY after 30 days would it be considered in the system as a return).
**Side note, if you purchase your phone online or customer care and return/exchange via online or customer care you will NOT be charged a restocking fee. However, if you purchase your phone online or customer care and return it to a retail store you will be charged the restocking fee.**
Yes, of course there are many work arounds. But I believe the OP needed to know the actual policy.
Now as far as the multiple plan changes in a bill cycle:
The billing system will allow 12 actual changes within a bill cycle. Here's the tricky part. Customer service and retail store uses a different system. Retail store will allow 3 changes and customer service will allow 3 changes. But, don't forget about online self serve. You are allowed 3 changes there as well. And....theres one more. Indirect locations (walmart, local agents, best buy). They use a different system. You can change 3 times in that billing system as well. Now, obviously doing more than 2 changes will make your bill a nightmare. And getting someone to fix that mess will be next to impossible.
Now on the ESN (serial number) changes:
You can switch ESN's as MANY times as you want. The systems lock billing on the 3rd swap within a bill cycle. BUT, you can run into serious issues here. Lets say you do the following: Env3 then Droid X, then env3, then Droid X. Well, the system will prorate you for 9.99 twice and Droid X twice. You are billed in advance so be prepared to be almost "double billed." Although the actual current feature you were charged for the previous cycle was the 9.99.
I post all of this so that the OP will have the "official" policy. Again, there are many "work arounds".