Customer When you do disconnect your tankless coil make sure you do not cap the lines going into the boiler. These need to stay open.
You can do a few things with an electric water heater and a tankless coil. First you can use the electric water heater as a storage tank and circulate water through the coil and into the water heater using an aquastat to turn the pump on and off. You do have to use a bronze or stainless steel pump and all copper , brass, or plastic fittings. This will give you storage and a constant temperature unlike the normal tankless coil dance around the shower too hot thing. This kind of a reverse indirect water heater. Another option would be an indirect water heater that uses your oil boiler to circulate water through a coil inside a storage tank. It would just be another zone on your hot water system.
If you want to abandon your tankless coil and install an electric water heater, you just take the cold water feeding your tankless coil and feed the cold water (right) side of the new water heater. The hot outlet from the tankless coil would be connected to the hot water side (left) of the water heater. Then you have to power it of course. they are normally 220 volts .
You could also preheat the water going into the water heater, but after a while the coil may get clogged up and your water pressure would be suffer. Also an indirect coil is normally 1/2" and you can pipe your water heater full size to get more volume of water.
Good Luck. plumber Tom