I'm involved in 3 campaigns, 2 near the end which will definitely wrap up under the existing rules, and the other will probably find a pausing/downtime point before changing.

In general I think we'll all migrate eventually, but need to actually see the rules and play with them before that happens. I expect a few one shots to help us figure it out.

In terms of your campaign, I'd stick with the existing rules since characters have already been created. If you find a rule you like in the 2024 rules just bring it up at the start of a session. For example maybe you like the new exhaustion rules, just be clear you're using those. Just be sure you're clear on which rules you're using and the players are never surprised by these rules.

The new subclasses are not compatible with existing classes in the current phb.

I have a group of new players who are 3 sessions into their game. The next session will be spent converting characters to 2024. I am also giving them a chance to change their characters no questions asked (some made some wild choices).

I have not looked at the new stuff at all yet because I am about to migrate my campaign world away from DND (probably to PF2e). The only thing I am currently waiting on is the last 3 sessions of my current 3 year run.

For me, the choice seems like it would be one of how much effort it would take to convert. If you are running mostly from pre-made adventures, I imagine it could be done with little fuss, but if it's an entire homebrew world that will certainly add complications.

Overall a campaign is a huge investment of time and resources, so my vote would be to do what makes you and your players comfortable. Being able to rely on a wealth of info on a system that has been around for many years can do a lot to put you at ease.

I am 3+ years in our current campaign so we will not be switching at this point. I will however likely do a one shot or two to get everyone up to speed. Might be what I do for our traditional Halloween one shot

I don't think my players want to buy new books so I'm not really planning to for now (like you, we just had our 2nd session this past weekend). Though it kinda sucked when someone cast Cloud of Daggers and I had to explain how both the "enters the area" text is (supposedly) cleaned up and better codified in 2024e, and that it can be moved now (from what I've heard). I might bring things in via houserule for situations like that.