Without a ruu matching your original cid there's no way (that i know of) to go back to a branded factory phone. But I'm pretty sure that if you just reset it to unbranded stock (there's a guide on how to do that somewhere, I'll try and find it if you don't), you won't have a problem with your warranty.
Since your phone'll be back to S-ON, locked bootloader (not relocked), original stock recovery, boot and rom, it'll be the same as a new one.
If yoy want to use your warranty, I'd suggest that, but if you're just worried about having messed with your phone by unbranding, rooting, using custom roms or custom recovery... well stop worrying, you're fine, evreything's ok (as long as you keep following the guides' instructions carefully !).