SMF is nice from an admin/mod standpoint but I don't think it's quite as user friendly. Plus, people here are so afraid of change it's almost painful to watch.
There would be issues with compatibility too. Even if we could transfer the old posts across, the formatting will be all messed up. The BBCode on SMF forums is both different and more difficult to get to work.
and maybe if they decide to keep invision. there are themes too for it. But remind this:
An open source forum software is easy to crack. WIth invision it isnt that easy!!
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I wouldn't mind a change, and in my opinion people might cry once the software is changed, but will shut up about it after a month or two, once they get used to it.
why don't you go for PHPBB? I've used that one for my clans forum for about 4-5 years, it's pretty basic and easy to use, but PHPBB also has alot of additional plugins to modify it to your liking.;)