Changing car number plate: assign or retain first?

[EVHead]

I’m assigning a personalised number plate to my car online. Do I do that first, then complete the form to retain my current plate, or vice versa?

I've read the online instructions but they’re not clear about which order to do this in!

You don’t want your current plate, so I think you assign your private plate to your car. When and if you sell the car and want to keep the plate, you put it on retention and then assign it to whatever car you get next. Each time you do this, there’s a charge, of course.