Screen Cracked. Need your help please

[mbaseball3]

So my girlfriend has a sprint touch diamond. She cracked the screen the other day with the phone in her back pocket when she bent over to pet her cat. I have read and I know the screen can be replaced. But my questions of concern are; if I go out and get a new screen and have the old one replaced, will we be able to retain the original warrenty? She does have the TEP. But since she just got the phone 4ish months ago, I've told her to save it for a worse accident that cant be fixed. And another question; where and who do you all recomend to purchase a good quality screen at and have it installed into phone? Thanks for all your help =D>