Upgrading Graphics.g4dn to GraphicsPro.g4dn fails with empty error message

I built a new Windows 10 Graphics.g4dn workspace, logged into it, and successfully installed and ran various software. After trying it out, I determined that I need more than 16GB of RAM for my task. The "Modify compute type" tab in the WorkSpaces console purports to let me upgrade the existing stopped instance to GraphicsPro.g4dn (64GB of RAM), but trying to save that change just gives me an empty error message of "If you continue to experience an issue contact the AWS Support Team on the community forums and via AWS Premium Support at: AWS Support Center." There's no other context explaining what the problem might be.

I did find this support page. It suggests that what I want to do is possible as long as the workspace is more than 6 hours old, which as of this morning this one is.