To be clear, you aren’t moving the rem to practise from their original place (ctrl+shift+m), you are making a new empty rem and either using ctrl+shift+enter to insert a portal bar and typing the rem you wish to practise or finding what you want to practice first, ctrl+shift+e to copy portal to clipboard and pasting it in the empty rem. Since the new rem will contain only portals, when you delete it you will only delete the portals and leave the original rem untouched. Of course, should you feel the need to edit or delete the actual some of them after practise, you may do so inside the portals.
Portals are, appropriately, just windows to other rem. You may open and close them at will, it only matters what you do to the actual stuff inside.
Here are some terrible pictures to illustrate the point.