I put my old plate on retention, just for that fact that all of the windows have the old number etched into them.

It costs around £25 to renew the retention every year and the DVLA send you a reminder so its not like you'd ever forget to do it and lose that number.

If you're not precious about the old number then its not really worth it as the others have said the DVLA will issue you with an appropriate number plate for when you take the private one off.

Mich