Converting ext disks to GUID partition scheme without erasing

I've got a few external hard drives that I've been using for a few years, and I just tried to encrypt them...only to be told by OS X that they need to have the GUID partition scheme. Is there a way to give them this without erasing their contents? They're big drives and close to full. Most of them are backup drives, so I'd really prefer not to erase them, nor do I want to buy a new drive just to use as a copy machine.