So I'm probably a little behind in upgrading my home networks domain schema to support Windows 2003 but better late than never!
The process itself was smooth enough once I'd corrected some problems on the machine but the upgrade logs were not the most helpful troubleshooting aid I've come across.
One particular error had me stumped for a few days:

"Error code: 0x57 Error message: The parameter is incorrect.."

No indication of which parameter it was but as it occurred when checking security descriptors and many blog articles refer to missing security ACL's on GPO's I had a look at those and sure enough, Enterprise admins was missing some rights so I fixed those up and....... the same problem. At this point I'd admit to a lot of head scratching. The event logs didn't shed much light until I realised that the security event logs were not accessible. Sure enough, somehow the ACL's on the security event logs had lost all their rights. Resetting these and then rebooting allowed the process to complete perfectly.