In most civilized areas there will have to be an asbestos abatement program sett up where the old boiler and related piping are removed.
Also considering natural gas is a dry fuel and oil is considered a wet fuel the chimney will have to have a certified chimney sweep make sure there is no residue soot in the flue /chimney which can cause an acidity reaction and eat away the lining.
#2 oil is rated between 138,000 - 140,000 per gallon Natural gas is 900 -1,200 BTU per cu foot so you had better find out the size of the oil burners nozzle now to get some idea of your heating needs
I cannot see how anyone can guess how big a one family boiler needs are as I recently installed a One million eight hundred thousand HB Smith boiler in a one family home
Is this boiler going to have a tank less coil for heating domestic hot water, what are the EPA rules in your are about oil tank removal?
Has anyone from the utility (gas supplier) mentioned changing just the burner rather then a total boiler replacement?
There are many factors to consider before switching from oil to gas such as pay back period and who is going to service the boiler / burner?
Is this system hydronic of vapor (steam)