If you're still locked in to your contract, then you cannot downgrade your plan. If you insist, you will incur pretermination fee.
For issues like this, you call 02-7301300 (recontracting hotline). Or 211 on your mobile phone and choose the option for recontracting.
Since you mentioned that you are already out of contract, you can definitely change your plan. Are you planning on getting a new handset or you just want to change your plan? If you want to get a new device, you need to contact a recontracting hotline. If not, you can try to contact an Account Specialist, you can send them a Tweet, at https://twitter.com/talk2globe
Hope this helps.