The NES and SNES hardware are too different for any kind of automatic conversion. The SNES is similar but much more powerful than the NES so porting games are probably not a problem for a professional programmer. But you still needs to know how to program games for the SNES, and it's not a beginner job, and not something that can be taught in a simple tutorial.
Early GBC games that had a grey plastic cart were backwards compatible with SGB. Clear plastic GBC games were not backwards compatible.
Wasn't that black carts though? DMG carts are (usually) grey, and GBC carts are black or clear depending on if they are DMG-compatible or not. And if they are DMG-compatible they usually also have some SGB enhancements, at least a custom SGB border.