I'm getting a Time frame that is currently wired for Di2 7970 (internal). I haven't received it yet, but how difficult is it usually to switch to another cable set, probably EPS?

I'm not exactly new to cycling, but I don't have a lot of experience with internal cabling, especially internal for electronic groups. How do you 'fish' the wires, are there difficult steps? Who does sell the plugs that make it look nice at the cable stops and other entry points?