According to this article by superuser , of UK:
http://www.technotes.co.uk/rename-outlook-2003-profile

" ... copying [the Profile] causes it to redownload all mail again, pain in the arse if you have a large mailbox .. "

The article then echoes Brad's instructions for editing the registry key.
The only addition I would make is to make an export/copy of the registry stub before changing any registry key data.