Upgrading from OS X Server to OS X (not server)

I am currently using an iMac with OS X 10.5.8 Server at work. We do not use this machine as a server. I would like to use the upgrade disc we purchased for 10.6.1 Snow Leopard, but am receiving a "Can't install OS X 10.6 over OS X Server" error message. I tried booting from the install disc. Still won't let me. My HD is already partitioned GUID Mac OS Extended (Journaled). Is there any other way to upgrade to Snow Leopard without erasing my HD?

I should add, I do have a retail disc for Leopard 10.5.1. Can I downgrade from OS X Server to Leopard first?

Is erasing my HD and doing a clean install the only option?

This should work:

Clone the internal drive to a backup external drive.
Clean install Snow Leopard onto the internal drive.
Migrate users and apps off the backup drive.

If this doesn't work as intended, you can always clone the external backup back to the internal drive and re-think it. But it should work.