Why not simply replace TCM ?

I believe the ecm & TCM is all in 1 module , so you possibly need to replace unit with
manual module or maybe reprogram current module to manual tranny os

Hi, you can put in a ecm from a car that has no automatic, you probably also have to
change the harness to a normal shift and maybe you must do a reflash. When you
leave it as it is you could get a check engine light and a engine that will run in limp
mode.