For Merchant ID (MID), there will not be any change, your MID will remain the same.
For Terminal ID (TID), there will be a new terminal ID issued for every new terminal.
You can find your MID and TID details via the Till Portal or on top of the receipt printed on your new Till Portable terminal.
Follow the steps outlined here to check your MID and TID details on the receipt of the Till Portable.
Or follow the steps outlined here to check your MID and TID details via the Till Portal.