The EPS plug is an 8pin connector which has 12V cables and GND cables.
The PCIe plug is an 8pin connector which has 12V cables and GND cables. It also has two sense pins which as per the spec can be connected to GND if unused.
The connectors are wired as follows:
So:
Chop off the EPS connector shells and throw them in the bin.
Then you buy new connector shells for the PCIe 6/8 connector (you could buy just the 6-pin connectors and get a 6-pin to 8-pin adapter cable, but no point if you are going to the trouble of recrimping).
Crimp on new contacts to the cables from which you removed the EPS connector
Plug in the new contacts in the corresponding locations on the PCIe connector shell.
That is all. No electronics need. You literally just need to make sure the 12V cables go to the places marked +12V and the GND cables go to the places marked GND.
Alternatively, buy an adapter cable. Search for "EPS12V to PCIe" and you will find several. This is such an example (no affiliation implied). It simply crosses the wires over, no electronic components required:
Image Source.