The general consensus is that there are few changes between the two compilers.

You have your own reasons for the change.

If your company still has a licence for the Keil compiler, you can verify your porting procedure at all times. I would prefer to minimise the edits, and implement the necessary changes via conditional macros.

You obviously have to test the identical functionality throughout this procedure.

You can use the standard tools to identify where macros are needed and used.

Until you have satisfactorily made the port, it is unwise to alter ANY program logic at all.

But the obvious question is:
Why reject a competent toolchain ?

David.