Can you use propane in a generator and then change it to natural gas?

I've done a little research in that area setting up a couple of natural gas generators for my home. What I found out is that propane (in some type of tank) and natural gas (coming from the utility company) have different delivery pressures.

The propane will require a regulator at the tank to match it to your natural gas service pressure levels. You can also expect propane to not be as energy efficient as natural gas, which means you'll have to use a larger volume of gas to deliver same amount of work, or output from your generator.

To switch between the two, you'll have to use different sized orifices to feed the generator. Depending on how your generator is set up to receive the gas (like a carburetor adapter plate) or some other type of feed system, you may have to do some modifications to be able to switch between the two.

Can it be done? Sure, but how much do you want to go through to be able to do it? If it's just a gas carburetor plate, you could probably stack them and switch the gas feeds as needed. If not, it might require some fancy Plumbing and maybe going as far as going to a machinist to have some parts made.