Personal number plates

It'll be reallocated to the car when the other plate's removed. Nothing else needed.

If you want to transfer it to a different car, instead of just having it "in limbo", then you need to transfer it in exactly the same way as any other plate transfer. If you then remove the plate you put onto that first car, it'd get a previously unissued age-related plate.

Thanks. Is there a fee for this?

No. You can't have a car with no plate on it, so if a plate is transferred off, one "appears" to fill the gap. Because plates are never re-used (unless actively transferred), the original is sitting there in the background.

They generally give the car back its original number. Or rather have done on the two occasions I've done this. So it pays to keep the original numberplates.