if its unlocked and was sprint branded. meaning it was originally a sprint phone yes its easy. But make sure esn is ok . if it is all you gotta do is unlock it. (which it should be if its on metro and was a sprint) then reflash sprints stock rom. activate the phone through sprint. then either leave it that way or flash a custom of your liking. btw below is a link to check esn to make sure its clear.
Online ESN/MEID Checker by deathtrip