Honestly? I'm can't see how they could pull it off. There's enough (probably a majority) of the BDs crowd that are really only there for Levellers and wouldn't be interested in anything else (or perhaps more fairly, would see the turning of the page as being a suitable time for themselves to move on to new things). So without Levellers you'd need to replace a majority of the crowd which is a tough ask. Probably can't do it gradually in this case, and would need a pretty special lineup to do it in a single year.
Can't personally see a scenario where BDs exists without Levellers as an anchor (using that word in both positive and negative senses).
No I've just read a lot about if Glastonbury got Chappell, Sabrina and Charli somehow it's absolutely huge. Neither of those are currently outright headlining something the size of Glastonbury in the UK and two of them are very much playing slot that are just sub level at Glastonbury as had been the case with previous bookings.