I need help with my htc diamond flashed to metropcs

[htc+dg]

i know just a bit on working with phones.
first question is.. I have an htc diamond flshed to metropcs. I belive I have a sprint stock rom. if i flash it with another rom would i have to take the phone to metro so they can flash it again?
this is probably a really dumb question but i just have my doubts..

also i'd like to know where can i find the dummy file to replace it "Windows\HTC_CM_Guardian.exe"

what i really want to do is have wap, mms, working with this phone
please help me