I-joists are stronger than sawn lumber. Span tables for Southern yellow pine limit 2X12 to 20' maximum span (by recollection, I'm out west now). Anything longer and you need to be 12" on center. The floor will be a bit of a trampoline regardless as you are maxing out the span. An 11 7/8" I-joist can span 24 feet or more depending on its rating (engineers specify the size and"weight of the I-joist). What happens for bearing walls at the stairs? Solid sawn lumber is different dimensionally than engineered wood and the two are consistent in that the dimensions match... that is, an 11 7/8" I joist has a matching parallam or LVL/LSL beam to carry the joisting and header at the stair opening. A 2X12 is 1 1/2" X 11 1/4" and a 6X12 is 5 1/2" X 11 1/4". Solid sawn lumber has to be much larger to carry similar loads. I would advise you not to sign the change order. 2X12 is cheaper than I-joists... Your spec is for a superior product and that was in the bid...leave it in.