I don't believe you can turn a chicken into a duck that way. Gas fired boilers will have burner trays running under the sections of the boilers where the gas burns as it comes out of the burner trays.

I have no experience with oil fired boilers, but they couldn't use burner trays like a gas fired boiler would because the oil is a liquid and would simply spill out of those burner trays onto the floor. My understanding is that oil fired boilers have a "pump" of sorts that pressurizes the oil so that it's injected into the boiler as a fine mist so that it burns both cleanly and completely. And, that difference alone is going to require a boiler of fundamentally different design than one that burns gas.

So, I really don't think you can change an oil fired boiler into one that would burn gas in an economical way. That is, I expect it would be cheaper to buy a gas fired boiler than convert an oil fired boiler into one that will burn gas.

Also, my understanding is that oil fired boilers are commonly used in rural areas that aren't serviced by gas utilities. Instead, heating oil companies deliver heating oil by truck to a storage tank on your property, and it's that heating oil you burn to keep your house warm. So, even if you could change the burner section to burn gas, if there's no gas service to your area, you'd have further expenses setting up some form of gas storage facility on your property.


But, in any event, a 20 year old cast iron boiler can last another 80 to 100 years if properly maintained. Longer if it's a popular brand and your local heating contractors stockpile parts for it. If this boiler were in Winnipeg, the economics of replacing it with a higher efficiency model might start to make economic sense based on the fuel savings. But, in Houston, where "winter" is the last two weeks of January when you might put on a sweater before going outside, replacing a perfectly good boiler with a high efficiency boiler to "save on fuel costs" is crazy. You're going to spend far more money on the new boiler than you're ever going to save in fuel costs.

And, on top of that, your average 20 year old boiler is pretty simple and every heating contractor in town is going to know how to repair it. A high efficiency boiler will have it's own computer to determine when and how it comes on and how it operates. So, if it's not working properly, only the heating companies in town that sell that make of boiler are going to be able to diagnose what's wrong with it. They know they know they have the customer over a barrel and charge accordingly for their services to repair it.

If I were in the OP's boots, I'd keep the existing boiler in their basement and keep the money that a new high efficiency boiler would cost in the bank so that when something crops up that really needs to be done, they have the money to do it. This way they're spending a pile of money in an effort to save $20 per year, and they leave themselves exposed to potentially having to borrow money if something else crops up that, unlike the boiler, really needs to be done.