Aye, thar be a few problems with this, as well.
Keeping in mind that the FH game server is connected to the site, that would mean that the staff would have to entirely revamp the way we make accounts, servers, etc. This forum in particular is designed to be connected to the game, and I don't think Proboards has that capability, and if it can, I don't know if it's strong enough to hold a community this large. I don't really see a need to uproot the forum, anyway.
Proboards can look nice, but the difference is that this site is more than merely a forum. Without it, no new accounts can be made, and as Bouncy said, there would still be a lot of issues with people not getting the memo about a new forum. Not to mention, the staff are already busy with enough chores and upkeep with the game, so making an entirely new forum, just for the sake of a new layout, might end up being more trouble than it's worth.
So.. It's just one of those "Don't fix it if it isn't broken" kind of deals. The input is appreciated, though, floof~