Also, the only way to provision a used phone is by doing a factory restore. Not all phones can do this, and it is always good to get the MSL+MSID from the CSR.
I've swapped well over 350 phones for people now and I have a specific formula that works, and works well. The only phones that have difficulty being swapped are ones that have an MEID starting with 2701131. Most Boost reps who are trained to know the MEIDs for Boost phones pick up on this instantly. Others just don't care and will let you know Sprint phone's may/may not work and they offer no support.
Also, almost every rep I've dealt with I tell them it's a used phone and instantly get the MSL+MSID, or if it's an 11 digit ESN I tell them it's a KRZR and they automatically give me the MSL+MSID. If I don't tell them, they always tell me to let it sit and wait for provisioning. Why mess with the hassle? My calls last no more than 4 minutes when I do have to call them in and I enjoy not having to go through all the BS of trying and hoping.