No idea on this matter yet. If that's Apple's artificial limitation, better not to do a "force conversion" at this moment. It's just a beta, not worth to find any complicated work around may become useless on next update. Also, may be the APFS can work on cMP, but only on a newly formatted partition. May be it can't boot after a manual conversion, who knows? Especially we have the 2nd firmware upgrade, IMO, that's a indication that Apple is trying to fix something.