If I purchase Easyjet Plus membership, can I also use the benefits associated with it for flights that I have already booked before purchasing Easyjet Plus membership, i.e. can I, for example, book a large carry-on bag and seat selection at a later date free of charge? I couldn't find any information about this on the internet, and Easyjet customer service also gave me evasive/unclear answers.

Does anyone have any experience with this and can report on it?