Transfer O2 number to new O2 sim

Hi.

I have a monthly contract with O2 to get 40gb for 25 euro, but I have a discount from UNIDAYS for O2 which gives 50gb for 23 euro and 100 euro gift card in Amazon.

The problem is they are sending a new sim with a new number, but I want to continue my existing phone number in the new tariff.

When I consultant to a O2 store they said only way to perform this action is the cancel my existing contract with O2 and port my existing number to another phone company and after that I should be able to port my number back to O2 for my new contract.

Is there a easier way to perform this action, preferably without porting my existing number to another company as it is already in O2?

Thanks!