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.
From the NME interview today with Oli Sykes:
What do you think of the rest of the Reading & Leeds line-up? You’re one of the biggest bands in the world, but you’re also the only band headlining?
“It’s all a bit beyond me now. I’m proper out of touch with what kids are listening to. It’s scary for us if I think about it. We still feel like the underdog and I’m constantly looking out thinking, ‘Is anyone going to come and see our band?’ It’s not Download, and it feels a bit more like each day of the line-up is aimed at a different type of listener. I have no idea if that’s a good thing or a bad thing, but R&L must know what they’re doing. I hope there will be more rock bands and artists from our scene on our day just to put my pessimistic mind at ease!”
Fulfil the wish of your headline act, R&L. Stick some bands on their day.