For the example of the Heckler 2020 originally fitted with the E8000 being upgraded to EP8, you are ironically better off if you started with the base-model bike. This is because the base-model had the (then new) SC-E7000 display and matching SW-E7000-L mode shifter fitted.
These 'low end parts' were actually newer tech than the SC-E8000 colour displays fitted to higher-up models when the Heckler first came out in early 2020. As a result:
The SC-E8000 display and matching mode switch are too old to work with the newer EP8 motor.
The base-model 2020 heckler has a display and mode switch that are fully compatible with the EP8 motor!
That saves quite a bit of money and hassle, since for a base-model Heckler you simply need the following to upgrade:
- EW-AD305 adapter x2 (connects the thicker old SD50 plugs in the Heckler to the new thinner SD300 plugs)
- EW-SD300 wire x2 (a couple of short 200mm lengths will suffice to make the connections to the new motor and speed sensor)
- DU-EP800 motor (hard to find but they do come up..)
- DC-EP800-A (or -B) motor cover (to cover the wiring connections on the EP8)
- DC-EP800-G bash guard (optional, to protect the underside of the lovely new motor)
- EP8 Crankset (Three options from Shimano themselves, plus aftermarket from E13 and Miranda) NOTE: E8000 cranks DO NOT FIT the EP8 !!!
- STUnlocker License (Use of a software tool enabling altering the circumference value is mandatory on a new EP8 motor. The default of 3000mm is useless on even a 29er, with assistance dying off at about 10mph!! Even if you do not intend to use the 'tuning' features of STUnlocker, you will need to at least change the circumference value to that of your rear wheel in order to achieve the 'standard' assistance speed ceiling for your market. In addition, you will need to enter the motor's angle of dangle (3 degrees for the Heckler) into the new motor's firmware.)
The parts list above will get you going assuming you already have a compatible display/mode switch. If not then you can add some newer components such as the SC-EM800 and the correct matching mode switch. There is also a lovely new budget display that combines the display and mode switch in one module (the SC-E5003) but for some bizarre reason this otherwise awesome display doesn't have Bluetooth.
The list above is also plug-and-play, but you can economise further if you are prepared to splice, solder and heat-shrink tube the SD50 to SD300 connections yourself (not a difficult task). You'll need to make a power connection and a speed-sensor connection onto the new EP8 ports.
If you choose to splice the wiring, you'll be able to purchase just one EW-SD300 wire and cut it in half, and you won't need the ED-AD305 adapters.
The stand out expensive bit is the EP8 itself (if you can even find it). The rest of the bits are very affordable indeed, especially if you splice the wiring and buy the base-model Shimano cranks (FC-EM600) which are more than fine.
The existing on/off sattelite switch is compatible with the new EP8, as is the Heckler's original battery - which plugs straight into the EP8.
The above information is for the Heckler specifically, but the same principle applies to any E8000 bike migrating to an EP8 motor.