No reson to do it . If you want a 23 , buy a 23. No need to "convert" a 21 to a 23. You would have to remove the B pillar , etc, it would be a major job, then you would end up with a car that you could never sell in the future because potential buyers would wonder why the VIN says RM21 , but no B pillar to be found. It would be assumed that the car was a rebody , because most people feel that nobody in their right mind would go through the trouble of converting a post car to a hardtop. Their are plenty of both to choose from ,
and no advantage or value diffrence from one to the other.

Greg