9-speed system to 11-speed drivetrain upgrade?
This page has the best answer I've found so far

http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-drivetrain-compatibility/

Reading between the lines, the cable pulls from 9spd to 11spd are probably close enough, but the derailleur ratios are significantly different. 1.4 vs 1.7. This means
that the 11 spd indexing in your new shifters will not work correctly with your old
derailleur.