I unlocked my phone a while back but have been having issues so Rogers has issued me a replacement phone and I have to send back my old phone. Is there a way to undo the unlock I did? I'm under the understanding that having unlocked my phone would void my warranty.

No. Sorry, once unlocked... always. It's unlikely they will even check.

Would seem pretty odd if it voided the warranty. Maybe the laws are different in Canada, but since RIM is supplying the unlock codes to carriers in the US, it would be hard for them to get out of a warranty claim because of that.