If you would like to convert all the user mail databases at once, do this:
load convert mail\*.nsf StdR4Mail mail45.ntf
This replaces the databases using the StdR4Mail template with the template in the mail45.ntf file.
However, this will delete all the user's private views or folders unless you tell the users to look at the properties on each of their private views and folders and make sure that "Do not allow design refresh/replace" is selected for each of these.
If you add the "-s" parameter, it will preserve any changes a user has made to the design:
load convert -s mail\*.nsf StdR4Mail mail45.ntf