While I agree with your edit point they could just make changes from set date - and do agree the change in primogeniture is new I'm suggesting they'll just fudge it going forward.
Princess Margaret husband was created Earl of Snowdon and Viscount Linley after marriage. As I understand Princess Ann and her husband were offered similar and turned it down. Her daughter Zara and son Peter are still known to grandchildren of the previous monarch.
So they'll likely offer some sort of title post marriage to Charlotte and she can either turn that down like Princess Ann or accept like Princess Margaret.
Louis will likely be offered a royal dukedom on marriage - it's usual practice - and can then follow his Uncle Edwards example and use lesser titles for kids.
Prince Philip was given Duke of Edinburgh title - Camilla didn't use Princess of Wales title but lesser one Duchess of Cornwall - giving new titles and using lesser ones if they choice are establish practises.
If Charlotte turns down titles for spouse and kids might look bit odd they are further up succession than Louis kids possibly using lesser titles - but expect press and thus public would just accept it with time - public would like Zara and Philip know who they were.
So I think they'll just announce to press titles of address or create new titles as they deem necessary.
I do wonder if they'll restrict royal dukedoms to that generation like Duke of Edinburgh with Prince Edward. Current practice is they stay Royal till they pass beyond the grandsons of a monarch.and then become a normal dukedom to pass down
So when they get a King with daughter as direct heir - upon marriage they'll likely just create a royal title/dukedom to pass down so the kids have titles so it won't be ever Miss and Mr.