Eric wrote:

But, the old forum should remain available as a “read only” archive.

That should be especially easy since the old forum’s URLs used the singular “forum”:

http://www.shga.com/forum/phpBB2/viewtopic.php?t=5196

While the new forum uses the plural form:

https://shga.com/forums/

So the old forum could continue to exist without conflicting with the new one. That would also preserve all of the external links into the Sylmar forum (which are now broken). Is there any reason that approach hasn’t been taken?