We can confirm that the second “Big White Lens” coming ahead of Photokina will be the Canon EF 600mm f/4L IS III. This will come along with the Canon EF 400mm f/2.8L IS III that we reported on last week.
We're upgrading the rumor rating on the Canon EF 400mm f/2.8L IS III to [CR3]
We can only speculate what the upgrades for the new lenses will be, but we're pretty sure weight reduction will be a big part of the equation, since these are the heaviest of the four most popular supertelephoto primes in Canon's lineup.
There's no word on whether or not the Canon EF 300mm f/2.8L IS II or the EF Canon 500mm f/4L IS II will receive updates soon after.