It sounds like you know what you want to do Ox, the opinions being expressed here are being met with sour responses. So just do whatever you want to do, its your site.
I will say that having paid for a lifetime membership means that the meat of your user base won't be subject to any paywall I would imagine as that just wouldn't be right. Therefore, how beneficial would this move be? I also think if you kept the main sports forum free, the other forums and chatbox aren't enough for most people to want to pay for. I could go argue politics on reddit for free. What other content would draw people in to a pay site?
And being even more brutally honest, the one content you provided after financial backing from your users, the podcast, you quit doing.
First it was a free site. Then it was a free site, but contributions welcome. Then it was a free site, but could get some cool swag with a donation. Then it was selling memberships and lifetime memberships. Somewhere along the way you asked for donations for the podcast, which netted you enough for some audio/visual gear, yet you quit after a brief period. Now you're talking about closing off a good portion of the board to a subscription. Just not sure where you're going with this and why the goal posts keep getting moved.