Good luck with that project. I've moved 2 large forums from a small, end-of life platform to Xenforo. Hard work but well worth it from an admin & end user perspective. Should be easier with a mainstream forum like phpBB especially if custom stuff can be replicated. Xenforo have regulars on the support forum who specialise in moving big forums like this. We did a dummy run first on a copy of the site & then moved the live site when we were happy with the process.