As far as is known @theglasswitch , the new especially for you pay monthly plan would replace your PAYG SIM. The price would certainly stay the same and you would even continue to use the same SIM card.
The only things you would need to do would be to set up a pay monthly account and change the App to pay monthly to manage the account.
Customer Services, live chat or the Social Team would clarify this for you. You can drop the Social Team a message here: Contact Us
