another possibility is to create the user with the "right" limited permissions, with the same password as 'sa' currently;
that should be easy to do , since everyone is logging in as SA, anyway, so everyone knows the password as it is.
After that is all set up then rename the 'sa' login, and then rename that new user to be 'sa';
an additional problem of course, if you need to upgrade to 2008, because i seem remember a thread Gail mentioned were in place upgrades fail on renamed sa, i think.
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