I was looking into this a few days ago and couldn't see anything about it.
My theory is basically this years festival is going to sell out regardless because there is no Glasto. It doesn't make sense for them to add in extra acts/stages. It would increase costs and bring in no extra revenue. can't think of many companies who would do that.
I'm very skeptical about the rumors, but the only was i could see it happening is with an increase in tickets or a bump in price. What I saw though looking at previous license details, is that when the numbers have been bumped up, the approval has been given quite soon before the festival, not the start of the year. So...maybe....but I still don't really see it.
The other thing is the size of the arena - it's changes a few times in the past an it would have to get bigger again to include an extra stage and more people. idk what the situation is at leeds, but I'd imagine it would be pretty easy at reading to add more camping over the river and move things about a bit.