I mentioned that I did this with my old community, Mighty No. 9 Universe in the thread listed in the OP. I don't think that ultimately killed my forum, it was already doomed since the official forums were public for everyone and not just backers of the Kickstarter project by the time I made the switch from paid to free hosting. I think if the host has all the bells and whistles for the community to thrive, and the community has consistent activity then the switch shouldn't ultimately kill the forum. It's okay to make that switch if you can't afford the costs for running your forum and still want it to remain online and under your control.