Changing floor plans that are offered are things the builders can do relatively easily, most sales contracts will state that the builder has the right to change things like that.
But going from SFR’s to Condo’s requires zoning and/or permit/govt approvals, which are much harder to come by. In general, when a builder does a “planned community” they are required to stick to their plan without MAJOR overhauls like changing from SFRs to condos. If a piece of land is not zoned for multi family dwellings, the buillder cannot simply change his/her mind and put them up… it’s a massive process to make that change.