Put a plug on the end of the wire

Is the circuit 20 Amp? How much current does it draw? Some microwaves draw a lot!

Just putting a plug on the end of the wire is NOT likely a legal or safe solution.
What microwave model are we talking about. Most likely you will have to hard wire it. You may be able to tap into the existing receptacle wiring (depending on just what it is).

The old microwave model was a JennAir JMC8130DDS. The new unit is a JMC2430DP00.

New Microwave

https://jennair.com/digitalassets/JMC2430DS/Installation%20Instruction_EN.pdf

Old Microwave

https://content.abt.com/documents/7446/JMC8130_install.pdf

It sounds like you need a single gang handy box cover with a 1/2" knockout and a 90 degree flexible metal conduit connector, assuming the flex is 1/2".

As long as there are no space constraints such as the junction box being in the way of the microwave going all the way in, you should be able to just take out the receptacle and hardwire it. With the old microwave being cord and plug connected, the junction box is probably in an adjacent cabinet instead of in the way? Since the circuit was fine for the other microwave, we probably don't need to worry whether or not it is a dedicated circuit. The circuit does need to have a way to lock the circuit in the off position since this eliminates the plugin, in order to meet codes.

422.31 Disconnection of Permanently Connected Appliances

(B) Appliances Rated over 300 Volt-Amperes or 1/8 Horsepower. For permanently connected appliances rated over 300 volt-amperes or 1/8 hp, the branch-circuit switch or circuit breaker shall be permitted to serve as the disconnecting means where the switch or circuit breaker is within sight from the appliance or is capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means shall be installed on or at the switch or circuit breaker used as the disconnecting means and shall remain in place with or without the lock installed.

Thank you Fred- is this something I could do as a novice or do you think an electrician would need to do this? And if so, is this something that is going to be labor intensive aka cost alot of money to convert?

A savvy amateur in a place where it is legal to do so can accomplish this. Note that unless you are in a single-family house, it is illegal in most places to dabble in electrical work.

It should be a small $ proposition to have an electrician handle it.

The OP never answered the question about circuit capacity, so I looked at the instructions for each oven which he posted. The old one uses 13 Amps and the new one 19.9, so I doubt that the old circuit has the capacity. Even so, is even a 20A breaker enough for a device which draws 19.9A?

I must have been half asleep on that part.

The new one is 240 volts and a convection oven. The old one was just a 120 volt microwave.

The microwave oven rated 120/240 volt, 20-amp, has 4 wires (L1, L2, N and G) in the flex conduit and should be connected to a 20-amp maximum rated circuit, overcurrent protected on both the L1 and L2 circuits.

This means that you will need an electrician to run a new cable to the unit that has one more wire in it, and put in a two pole breaker :(

I even remember seeing that before, but was in the middle of being interrupted :-/

Mtvhike, 3250 watts ÷ 240 volts = 13.54 amps

I don't know if they were using new math, or if that 19.9 is just their strange way of saying max 20 amp breaker. Either the watts or amps isn't telling the whole story. In the end, an electrician needs to refer to the nameplate on the unit.

I was just reading the documentation posted here.

Yes, they may not be including a small motor in the 3250 watts, but computing the total load and other % factors into the 19.9 volts already.