BMG Extruder upgrade questions

Sorry for my ignorance here. Hope someone can help.

I use my 3D printer occasionally, so I have to re-learn many things when I upgrade or make changes. I currently have a Geeetech "Prusa I3 X" and am upgrading to a BMG extruder - so I'm moving from a direct drive to a 3:1 gear ratio. In printing carriage parts, I've had to flip the drive belt, so I also need to change the direction of the X motor.

I'm always a bit confused which settings I change using the Arduino configuration.h file, what I change in the Slicer, and what I change in the software settings. I currently use Repetier-Host with Prusa slicer.

So with this basic confusion, where do I change:

1. The new settings for the BMG extruder (extrusion ratio/speed), XY offsets (from the endstops) for the new nozzle location?
*edit: I think I found the Extuder E-step value:
#define DEFAULT_AXIS_STEPS_PER_UNIT { 80, 80, 2560, 415 } in configuration.h - 415 is for BMG.

2. Change the direction of the X motor?

Thanks for the help. Or if you can point me to the right resource - that would be great.

I think this is where my confusion is: The EEPROM can be changed in many ways - onscreen menu, through the 3Dprinting software, and by uploading Arduino code. Is that about right?

Curious why you're upgrading.
I've never changed an extruder in any if my 6 printers, and I generally print faster than mist people.
Getting settings and filament right makes a lot more difference.
The BMG style are okay, but no real improvement on a traditional style mk8

You can't shortcut learning how to use slicers and understand how different filaments work by just changing hardware
And until you do learn those those things, nothing you do to the printer will make any real difference.