Is it possible to change a cross listed course code after completing it. For example I took Enviro Sc 2EI3 but since I want to do a minor Geog I want it to be Geog 2EI3 (There are both cross listed against each other). Any help would be much appreciated?

It's the same course just listed under different subjects/departments. I don't think you'd be allowed to register for it again. :/
I would contact an academic advisor though and see if the course can count towards your minor anyway.