Is there a way that I, a user who has been with Curve from the beginning, can get onto Curve Black (without subscription)?
Unfortunately not. The only options for you are to upgrade based on the tiers currently available.
Are you a Curve blue user then? If so the above applies if you are already a Curve black user (before the 27/1/19) then you will be grandfathered to Curve legacy black as it’s now called.