cbwjm
Seb-wejem
You'll have to wait until we hear more about how 2014 PCs interact with the 2024 rules. My guess is that spell lists are unique to the year so if using the old warlock, you use the old warlock spells, not the new one.So I sit down with my 2014 warlock and my buddy sits down with his 2024 warlock... he takes blade pact I take chain lock. I hit the level where I get a new cantrip and do what I do now... google "Warlock cantrips" and now a new one comes up 'pact of the blade' and it is exactly what my buddy is doing and it looks fun but it is just a cantrip... so I take it.
NOW, explain why the new book that has a new warlock cantrip cant be taken as a warlock cantrip?
Also, in your hypothetical scenario, if the DM turns around and says that cantrip is for thr 2024 warlock, you'll need to select another, are you really going to argue the point? I feel like most reasonable people would accept it and move on to selecting a new cantrip.