I moved one of my production sites from a NT 4 DC to a Windoes 2000 member server. It went very smoothly.
Before starting, I would download the SMS Security Essentials White Paper. This describes the functions of the various accounts in excruciating detail. I would also work through the SMS Recovery web site.
Is your site server a PDC of it's own resource domain? Is it part of a hierarchy (with parent and/or child sites)?
If not, I don't think that the move would be particularly painful. However, it will need careful planning.
I would implement all of the optional accounts that reduce reliance on the SMS generated accounts (Client Connection Accounts primarily).
I personally use a complete test environment. This allows me to pilot changes before I impact my production sites.
Chuck