My experience with this: my forum software was really really crappy, and free. There was no way to convert it without doing some expensive work.
Ultimately I ended up taking a pure html copy of the old forums, more than ten years worth. It's completely non functional as a forum, and unfortunately all the DATES are gone from everything. You have to use Google to search the non-functional archive. If you know what you are looking for you'll probably find it.
But I have it, and NABBLE is out of my life forever.
XF is super easy to set up. I think it's $175 the first year and $55 each year after. Support is good, and there are tons of active members who help. XF is pretty good with bug fixes, but they assume in the forums that you know everything about their amazing product. It can be humbling.
Personally, I think if Tony applied himself here, he could get a new Xenforo with all the old content and users intact. If it turns out I'm wrong, then you could take the bail out I did, with the html backup.
Take a look at the link to the NYSB Nabble Archives at the bottom of the page:
Our Northeast Ski Forums cover the Adirondacks, Catskills, Tug Hill, Central NY, VT. Skiing, trip reports, weather, conditions, gear and life. All of it baby.
Not fantastic, but way better than losing it.