Under the assumption CR's rumor is true
Difference of v2 as compared to v1
- More lens elements and groups
- Differing Focus adjustment
- Shorter focusing distance by 0.82m/2.7ft
- Zoom system possible twist-type rather than push-pull
- Larger Diameter x Length, Weight 2mm x 4mm; 190g
- 2 more stops of IS
- 16 year gap between the introduction of the v1 & v2
- $500 more
- Inclusion of IS Mode 3
http://www.usa.canon.com/cusa/consumer/standard_display/Lens_Advantage_IS
Version 1 vs 2
v1
Focal Length & Maximum Aperture 100-400mm 1:4.5-5.6
Lens Construction 17 elements in 14 groups
Diagonal Angle of View 24° - 6° 10'
Focus Adjustment Rear focusing system with USM
Closest Focusing Distance 1.8m/ 5.9 ft.
Zoom System Linear extension Type
Filter Size 77mm
Max. Diameter x Length, Weight 3.6" x 7.4", 3.1 lbs. / 92mm x 189mm, 1,380g
2-stops of IS
Introduced 1998
Price $1,699
v2
Focal Length & Maximum Aperture: 100-400mm 1:4.5-5.6
Lens Construction: 21 elements in 16 groups
Diagonal Angle of View: 24º-6’10’
Focus Adjustment: Inner focus system / USM
Closest Focusing Distance: 3.2 ft. / 0.98m
Filter Size: 77mm
Max Diameter x Length, Weight: 3.7 in. x 7.6 in. / 94mm x 193mm; 3.46 lbs. / 1,570g
4-stops of IS
Introduced 2014
Price $2,199
Edit: Added more "complete" specs