Very pleasingly, I've managed to upgrade my forum to the latest phpbb with everything intact!
However, I've encountered one issue so far and I would very much appreciate any advice.
In the ACP, when I click onto Users and Groups, it opens this error page:
This relates to a mod I once had installed on 3.0.x:
I'm wondering where on earth it's still affecting the board, as the only remaining files from the original board, per installation instructions, are in images/, files/, store/ and config.php.
I found a folder for Alternate Login in /store/mods/, deleted that, but to no avail.
I'm also looking through the manual installation instructions for that mod to see if I can trace any edits to files in that folder, but I can't see anything yet.