Yes. You need to contact CircleCI to request using the resource_class feature, even if it’s a downgrade.
I made a feature request to allow this for anyone, but it’s unlikely to be a priority. I suspect that in Docker, there is not a RAM saving from a CircleCI perspective; I believe Docker containers do not reserve the whole of their maximum memory - only Machines (VMs) do that.
(Aside: I couldn’t get the Ideas page to work for me, so it didn’t go into the proper list anyway. Too many conflicts with NoScript and/or Firefox, IIRC).