I should think everybody is expecting to receive some mail which uses old stamps and has been surcharged - or which has been allowed through when it should have been surcharged.
I've started a separate post on my Postal History Blog and look forward to seeing your comments and examples there.
Visit In the post after the Machin and Country definitives invalidation on 31 July 2023
Here's the first one, sent Bristol - Devon 16 August 2023
I'll note here when the other post is updated, but please follow there as well.
UPDATE 29 August - more examples have been added including correctly surcharged cover, and more which should have been but were not.