Don't forget that as standard the e8000 controller is normally configured for none di2 gears, not Di2. So it won't work if you just plug the cable into the display and the other into the motor outlet.
You need to either take it to a shop so they can plug it in and change the setting to Di2, or install Stepsunlocker for android or IOS and set the system to Di2 under the "Gears" setting. I think, but I'm not 100% sure, you can change this with the base version (less than €2), otherwise you need a licence, which is €30 - so still not the end of the world. There are probably other apps which let you do this also, but I'm familiar with that one.
If the Bike is a Di2 configured bike, you can swap to conventional Mech without changing settings - it works out the Di2 isn't plugged in and auto switches to normal (so if you were going away and carried a spare cable,shifter,mech - you could swap if you broke something). Likewise, you can change the configuration before you do the physical change and it shouldn't affect anything.