I won't pretend to know the potentially complicating, expense adding details.
But this was just the first one that popped up in a google search, one of many.
https://profprojects.com/migrate-ubb-threads-to-xenforo/
100,000 users and 1,000,000 posts migrated from UBB to Xenfro for $795. PM might have more than a million posts? Definitely doesn't have a million worth saving though. Script could yank out all the one sentence replies, for instance, trim it down a lot, in a hurry.
At the end of the day, migration is just a custom script. Outfits like the above have already written them and just need to plug in the variables specific to a given forum for the migration. Then just run it. This isn't custom programming. It's off the shelf stuff.
But, like I said, I don't know any of the particulars of PM. I do know, a 4x4 forum I'm on, did this same migration a couple years ago and it's been a huge success. They have more than 20 years worth of archives, too. It's just a local 4x4 board, but it has way more activity than PM does.
I don't know that a software upgrade will "save" PM. PM has shot itself in the Johnson so many times I cringe to even think about it. But I'm pretty sure PM is not going to gain any momentum or become more vibrant without an upgrade. Could trudge along like it is for a long time, but it's got zero long term viability without some badly needed modernizing.
Get a few quotes on the migration. You might be pleasantly surprised how affordable it might actually be.
Good choice on xenforo, too, BTW...
- DAA