While it might not sound like much the policy WILL be retroactive, so downloads from before the start date will be counted. Even unofficial/pirated copies. Given the time you've been working on this game it's unrealistic to expect an engine switch. I know some devs are switching to Godot. I don't know if it's similar enough to unity that you could take your skill with you but it may be worth changing over when ever you go back to your old games.
Having said all this with the backlash Unity have been getting even as a soulless corporation I can't imagine the current plan to remain in place. Time will tell Jan. 1st.