It's been years since I played around with runtimes, but I suspect that you need to edit and use the original file that the first runtime was made from, and NOT the one from inside the runtime folder which would have been placed there when compiling the v12 runtime.
I hope you are not correct about that. I have made lots of changes to file using the file in the compiled runtime using FMPA12, so using original file may pose a problem.
Thanks for your help
Try using a different file extension when creating the new runtime.
Thanks for suggestions and my apologies for delay.
The runtime is used on a computers at a very large company and I realised they are still on XP which is not supported in v13,