Switch for electric on that 'old' one (9E) would be on backside on water heater on junction box that covers the electric screw in element.
Using the 12V DC that feeds the old Propane ON/OFF switch is HOW the new switch should be wired.
Both electric and Propane ON/OFF switches use the same 12V DC Power source.
You will need to run a NEW DC wire from electric on/off switch to the 'WHITE OEM Wire' on NEW WH.
The OLD 120V AC Power form a circuit breaker is still used and connects to the DC Relay which then has AC wires that go to element....ALL on backside of water heater
All of this is explained in the conversion info in manual linked