Originally Posted by
Bart L
I know I can probably keep the 9 speed derailleur and make it work with the 11 speed cassette
No you can't. Assuming Shimano components, they changed the rear derailleur geometry and cable pull requirements going from 8/9/10-speed to 11-speed. So along with the new cassette, chain and shifters you will need a new rear derailleur too. The 11-speed chain will work with your 9-speed crank.