Thank you Peter. That's the ticket.
The layers actually map from the old to the new sequentially, not by name, so all I had to do was change everything on my Mech12 to Mech8 then update the PCB with a right click on the library item in question. It magically showed up on the Dimensions Top layer in the new PCB. I thought it was going to be more diabolical than that!
I used Mech12 & Mech13 as my top and bottom assembly drawing layers in Altium for many years. All my footprint libraries have "data" on these layers.
Now that I've switched to Circuit Studio anything on Mech12 in the PCB library editor comes in on the "Dimensions Bottom" layer in the CS PCB editor. I think I want it on the CS "Assembly Top" layer. There is no Mech12 in the layers list in the CS PCB editor, nor is there a "Assembly Top" layer in the CS PCB library editor.
So how can I get this Mech12 library data on the PCB "Assembly Top" layer?