Deselect the cell under rooms option, within a few seconds the prisoner will move to another cell, then make it a cell again ready for another prisoner,
another option is to move the cell improvements from one cell to another, like the tv. radio etc,
either way is a hassle but i have a mixture of cells with qualities of 8 and 10 and when a prisoner needs a higher quality cell i just deselect the cell he's in.
Unfortunately when a prisoner goes to solitary the guards put him in a new cell when he gets out and they can put him in a cell that doesn't meet his needs.