Replacing turfs or areas with any other object type would be impossible. This restricts your suggestion to specifically being able to use find/replace to replace objs with mobs and mobs with objs, which is reasonable but I honestly don't see how it would actually be useful.

Because areas do not have a single instance per location, they instead generally have one instance per type and simply contain a list of turfs in them. As for turfs, replacing them would require creating a new turf in their place as there must always be exactly one turf at every map coordinate, which makes the concept of "replacing" them with anything but another turf silly.

There are convolutions of the term "replace" which are technically feasible (IE: "replacing" an area means deleting that area and filling every turf which was in its contents with a new object, as well as changing its loc to the base /area, and "replacing" a turf means deleting that turf, creating a new /turf in its location, and creating a new object on that new turf), however, none of these are strictly "replacing" in the sense that a reasonable person would use. Furthermore, none of them would even be particularly useful.

If you feel it would be helpful to you, might I suggest you actually figure out what different object types are actually used for? They aren't something you can really say, "Whoops, I meant to have something else instead!" except possibly objs/mobs.