In the parts of Peabody where gas mains run, oil-to-gas conversion is a live option when an oil boiler reaches end of life — but it's a when-the-boiler-dies decision, not a mid-life one. A sound oil boiler with years left is almost never worth replacing early just to change fuels. The math turns when the heat exchanger is suspect or the tank needs replacing: at that point, compare conversion against a new oil unit honestly, including chimney and gas-line work.
Where mains don't reach — parts of South Peabody and the older streets — conversion isn't on the table, and the right move is maintaining the oil system properly: annual service, clean fuel, sound tank. A well-kept oil boiler is a perfectly good heater; neglect is what gives oil its reputation.