Oh, I see. The script runs during installation and creates the stored procedure. (Sorry for my ignorance. :) After changing the stored procedure, it installed just fine.
I wanted to let you know about something else that happened, though, in case it's relevant. I wanted to change the minimum required password length, which I have done in previous DNN versions via the properties of the "website" folder in IIS services manager, ASP.net tab → Edit Configuration → Authentication → Manage Providers. As soon as I had done this, I started getting errors when attempting to browse the site, saying that the Active Directory authentication provider could not be found. I had to go to a backup web.config in order to be able to do a standard login again, and then reinstall the AD provider. I finally figured out that if I just changed minRequiredPasswordLength on the AspNetSqlMembershipProvider line in web.config, it worked with no errors.
Thanks for your help!