While it true we don’t have all information yet, we do have a good idea of where things of going: we have what WotC has said (CRs will not change, monsters will be better balanced, they are making changes to make them more interesting/ easier to play) and the changes they have made in recent books (which they said are previews of monster changes) which match these goals. Fizban’s, Bigby’s, and Planescape display this philosophy well, MMotM too.
Basically, monsters will get minor tweaks to:
- Make them reflect their CR more accurately (if needed)
- Make them easier to play (removing slot based spellcasting)
- Make them more interesting (one unique thing)
Those changes (some will not even need changes to comply) make the monsters different, but don’t (in theory) change the challenge of the monster. That is why (in theory) you should be able to use either ‘14 or ‘24 monsters in adventures from before or after 2024. They do not need conversion or swapping out because they are the same CR.
Now, it remains to be seen if they can execute that plan, but that is clearly the intent. So I think that your idea that monsters have to swapped out like 1e to 5e is clearly not the intent and has less supporting evidence