bobbel has a good and easy idea to visually transition from one object to another. As for when to load/delete objects, I'd prefer pre-allocation.
Load both objects into memory, using only one of them at a time; use EXCLUDE OBJECT ON on the other one. Then to switch you EXCLUDE OBJECT OFF of the second one, position/rotate it to the first one's positions, and EXCLUDE OBJECT ON on the first one.
Loading and deleting lots of objects per second has a serious tax on some hardware, as I learned with a fire particle emitter I made. So instead of dynamically loading/deleting the particles, I learned to load up all of them at once, and only EXCLUDE OBJECT OFF on the number of particles that were needed at any point in time, then EXCLUDE OBJECT ON later when it had done its duty.
Remember those old guys? They made epic renders, I think one of them was called DaVinci, and all they used was MS Paint. Sometimes it's just skill....
