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!
This thread seems to contain an interesting clue to a serious problem we have had since converting to CS months ago: Layer names seem to be editable, but at various times you find out that they really are not editable. Instead, there is some kind of list of 'absolute' layer names that manifests itself at times as in the original post here. For us, the biggest problem with that is that ODB++ output files and folders have names with UPPERCASE characters. This is expressly forbidden in the ODB++ Standard, and the file structure regularly crashes ODB++ viewers. We have to manually edit all names with uppercase characters before submitting ODB++ files to our contract manufacturers, and this is a tedious process. The reason for my replying to this thread, is that I see that Peter Barnard has actually replied to the message here, and I am desperately hoping for an actual reply myself. To date, no one from CS support has ever contacted me about this issue.
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?