Can i use a previously owned sprint iphone with my own sprint account?

My friend is selling his sprint iphone 4s phone and i have a sprint account with a iphone 3gs, i'm planning to give my iphone 3gs to my brother and buy my friends iphone 4s to myself but on the account matter, can i switch my 3gs account to my new 4s and yes i know that i'm able to open another plan for my old 3gs for my brother o the same account but i just wanna know if there's any problem.
Note: does my friend has to cancel his account to that phone and the phopne can connect to mine?

Iphone 3gs can't work on sprint.
you can use the 4s on your account if its off contract and the esn is clean.

I think you can, but only if it has a clean esn, if its bad you wouldnt be able to connect it unless you have someone clear it

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