Now live with no issues so far. If it helps or reminds anyone, the process for me is:
Create a site subfolder called \forum1 and copy in the new forum files.
Take the following actions:
* Copy in my old customised CSS files (first check there are no new entries)
* Copy in my old database_connection.asp
* Copy in my old header.asp and footer.asp
* Copy in my old \uploads folder and set permissions to allow uploads
* Copy in my old \uploadsa and \uploadsi folders from previous versions
* Switch off Birthdays (and reinstate any other customisation you have)
* Test using \forum1
* Go live by renaming \forum to \forumold and \forum1 to \forum