Do not 100% quote me on which of the 2 big white lenses will be announced. It could be the 300 & 400 or 500& 600. I'm still working that out. The 300 f/2.8L IS seems to be the #1 candidate.
I'm still working on when.
I own a lens rental company and buy these big things. I own all the Canon supertelephotos. I was going to buy one more of 2 of the lenses and was told some things.
Big White Lenses Being Replaced
300 f/2.8L IS
400 f/2.8L IS
500 f/4L IS
600 f/4L IS
Now what may happen is only 2 of them being replaced at one time. The most likely candidates are the 300 f/2.8L IS & 600 f/4L IS.
What will be upgraded?
The obvious upgrades will be the IS systems and coatings on the elements.
The BIG feature will be significant weight reduction. How much? I can't say, but that's what I'm told.
How much will they be?
If you want an ultra sharp out of this world supertelephoto from Canon, just buy any of the current ones. I own them all and they're great. The new ones will come it AT LEAST 25% more expensive.
This is why it's [CR2.5], I don't have a definitive time-frame. I'm told “soon”.
Lens Hood Design
I for one think the current lens hood design is annoying at the best of times. The one thing I do like is that the fastening mechanism is reliable, I have yet to see one break. If they do redesign the hood locking mechanism, lets hope it holds up over time.
What about the 200 f/2L IS & 800 f/5.6L IS?
I'm not sure if either of these are included in the supertelephoto replacement. Outside of perhaps a new hood, I can't see them included.
400 f/4 DO IS?
I've heard nothing about a replacement for this lens.