Hi @Ashling_52775 and welcome to 3Community. Do you know if you are in contract? If you are moving to SIM Only will cause you to receive a cancel contract fee. If you're not in contract you'll be able to change you're plan through your My3 account, once you've logged into your My3 account pop down to the Your Plan section and click 'Change Plan'.
Here you'll see all plans which are compatible with your current plan. If the plan you are looking to change to isn't displayed or you'd like more info on a specific plan please PM us with your name, number, full address and date of birth.
