the efficiency is pretty good already; you may have slightly better efficiency on gas, but your savings will come from the difference in the price of the fuel, not from replacing the boiler. Which is considerable.
Some boilers can be converted from oil fired to gas fired (not all); you might look into that first.
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
If the current steam setup is well matched to the heating load (EDR) the savings will be the difference in fuel costs, last year oil was running about $3.50 and NG in the Boston area was about $2.20 for the equivalent amount of gas (1.4 therms). We won't know what the cost is till the gas company posts this years winter rates that take effect at the end of October.
If that steam boiler is oversized you can dial the output down to match the load by drilling the correct orifice size for the input BTU you need and that can save some fuel.
Smith G8-3 with EZ Gas @ 84,200 BTU, Single pipe steam