No, you can't transfer an existing iD Mobile number to a different iD Mobile plan.
This rule applies even if you have a PAC code, as these are only for switching between different networks.
If you've bought a new plan by mistake (instead of upgrading your current one), you'll need to return the new order. Check out the Community article below for more helpful information:
Please note: Moving a number is different to a Plan Change or Upgrade. If you want to change your allowance or phone on your existing number, check out our guides on Upgrading and Changing Plans.