True - but are any of them from breakaways/splinter groups?
I have seen it happen on several forums (mostly HiFi / AV related) where a core, key set of users get pissed off with the administration so setup on their own, often taking a large user base with them.
If you can somehow make these key "players" aware of the existence of microcosm and its ease of setup then I think you could grab a large number of users quite quickly.
A lot of forums use google ads - could you take out a small test campaign to target getting adds on forums along the lines of "microcosm: create your own awesome, mobile friendly forum in under 5 minutes, for free! "
If it works, this has the benefit of not requiring a migration but still having a strong initial userbase...