Yes you would need to purchase support for the 50 additional licenses.
If you try to contact TAC without the correct support coverage, They will refer you back to your VAR to remedy it before assisting you.
While i haven't attempted to upgrade firmware on a ZD that is only half supported, I would think it should deny you the same as if it wasn't supported at all.
Note that while you still have the option to purchase support for TAC assistance ZD3000 has already passed it End of Maintenance date, and will no long receive firmware patches.
I would suggest that you consider transitioning to the smart zone platform. your ongoing support costs would likely be cheaper (especially if virtual).