Sorry to be a smart-ass about this... Technically you can un-root without wiping the phone, it's just a royal pain. Lots of steps involved. Basically you would flash a stock, non-rooted ROM without doing a wipe (so your /data would still be intact), then flash an S-ON bootloader, and then flash the original recovery image. You'd have to be very familiar with using adb, and you'd have to make sure you use all the right files, and there's no way I'm going to make a set of instructions for doing so because I can't imagine a practical purpose. After all, the only time you benefit from un-rooting is when you're returning the phone, and you should really wipe it if you're returning it anyway.