tmlbll, There is no limit on the amount of phone swaps you can perform on sprint.com. Though it is probably best to limit the exchanges to 1 per week if you plan on changing phone frequently. If you have any issues performing the swap or getting the phone activated please call sprint. One thing I would like to advised you of is with a Blackberry do those do have to be reset to factory default in order for it to program the new information. You will more then likely have to contact Sprint for assistance on that. You can choose to use the chat option, go to a Sprint store or call Sprint at 888-211-4727 please do not call from either of the devices you are swapping.
@tmlbll - Any phone swap through Sprint.com that involves a phone that was previously active on another account or phone number will require a call into Care or visit to a Sprint store. The swap can be performed online, although as lilgeekygirl mentioned the Blackberry would need a hard reset performed and the code to perform this is not available online at this time.
If you are swapping to an old phone you had on your number before and had not been active on another number, you should be ok other than phones that require certain plans like the Blackberry.
You can swap your phone on sprint.com/activate, in a store, or over the phone with support. If you are syncing with gmail, your contacts will transfer over once you sign into the gmail account on the new android phone. If not, you can export your contacts to the sd card and import them to the new phone.