I didn't realise this. I logged in to my online AIB to investigate...... Below from my AIB online it says "Should you wish to amend your Direct Debit details, you should contact your Originator directly.
AIB do not have the authority to amend the details of your Direct Debit mandate."
There is a cancel option however....
Cancel
After tentatively selecting the cancel option for one of my DD's, I got the message below... and I didn't proceed.
Important Information
By selecting "Confirm", you are authorising AIB to cancel the above direct debit and no further payments will be made by us under this mandate. It is your responsibility to inform the originator that you have cancelled the direct debit and to make any alternative payment arrangements necessary.
To ensure that associated payments are not processed, you should cancel your direct debit no later than 21:00 hours on the business day prior to the payment due date.
So I can't amend but I can stop/cancel. It makes sense not to amend for me as I can query a bill in time before the payment is taken and have the right to cancel the payment/mandate. A supplier such as Three will obviously amend monthly payments due as my mobile phone bill naturally varies every month.