I'm assuming you've issued your BP and 1D is free and you want to move there to be adjacent to 1F? I am also assuming this is a CE flight on an Airbus so 1A-C and 1D-F are the seat pairs, and someone has snagged 1C whilst you were checking yourself in.

As far as I know you have to either wait to get to the airport and change then OR call up and ask to be offloaded (check in cancelled) and re-do OLCI. Phone agents CAN 'offload' you, but what I'm not 100% sure about is whether you can re-do OLCI or if you'd need to check in in person - I think you'd be able to re-do it but would want that confirming!

I've not used the new lounges in T3 so will let others comment - CX gets good reviews.