How do I change from Master Boot Record to GUID scheme?

I've downloaded Lion and the first "complaint" the installation software has is that neither my Mac hard drive nor Time Machine drive uses the GUID partition scheme. The software instructs me to use Disk Utility to select the appropriate format but Disk Utility states that I can't re-size the partition because it uses the Master Boot Record scheme. Further, it tells me that the drive can't be erased because it's the start-up disk. Any ideas on what I should do?

I have Snow Leopard 10.6.8 installed on my MacBook (Core 2 Duo, 4 GB). The Mac HD has 220GB available.

Jeez... I waited for Lion to be reasonably fault-free and carefully looked at Lion's requirements only to find out too late about this GUID scheme business. I haven't seen anything about this requirement until I tried to install. I have great fear of re-formatting the drive as that always seems to be the beginning of many problems that take months to resolve. Maybe the choices are to forget Lion or get a new drive and format it to GUID before restoring from my Time Machine

Any suggestions will be appreciated.