Check with the boiler manufacturer or company which installed the unit. My experience with any of the conversions has been a dismal experience at best. They tend to use different technologies considering the different heating properties of the fuels (oil heat is considered a "slow" expanding type of flame to heat a broad surface whereas NG is used to "spot heat" an area and conduction is used to spread the heat) and usually the firebox has to be remade.

This is old experience (40 years ago) so depending on the age of your boiler it may not be applicable.

I'd go for the conversion because you can easily convert it to propane with a simple orifice change down the road.