It was supposed to be a simple case of copying some files from here to there and then running an 'upgrade' script, but of course it became an 8 hour ordeal just to get things into a state where it would even allow me to run the upgrade script, with just blank failure shown on screen and mostly unhelpful messages if at all.
Emails to and from my host and a support request with the Invision forum people, plus me baffooning around trying to change random settings that I read online had helped for a bloke called Keith in a similar but different situation and here we are!
It's all upgraded so hopefully it now works flawlessly in every respect.
What definitely no longer works in the rest of the predecimal.com site (from the root, i.e. without the /forum bit). That wasn't really doing much and the aim is to now move the forum over to the root so that predecimal.com is just the forum. I'll also change that old banner stuff at the top and maybe add some extra info about predecimal.com, but first, let's see how all this new software and stuff works.