I've also got it working while still using the $F. My file path was "testsmoke.$F.vdb" instead of "testsmoke_$F4.vdb" which wasn't working and I set the Houdini project to have a start frame of 0 instead of its default of 1. Those two things got it working for me.

Existing owners will have perpetual ownership of their licenses right?

Yes. ORC is currently the only opt-in feature of Octane 3.x that must have a monthly subscription to use, as it requires ongoing service costs independent of your existing Octane licenses to cover cloud transfer, estimates and ...