Conversion 9 spd to 10 spd I'm thinking of upgrading from a 9 speed (11-34T cassette) to 10 speed.  Scoped out a few components and 
1) Shimano Deore XT CS-M771 10-Speed Cassette 11-36T 10-speed HG cassette
2) Shimano Dyna-Sys, super-narrow 10-speed HG chains (hg95)
3) Shimano Ultegra 6700 Shifter (for 10 speed)

I have an existing Deore XT rear long cage derailleur used to support 9 spped 11-34T cassette.  Two part question:  

A) can existing rear derailleur support HyperGlide chain and cassette?
B) any other comments on compatibility of proposed new components or suggestions?
The first thing to check is that your rear wheel can take a 10 speed cassette. If this is the case, then you can move to step two, which is picking compatible rear derailleurs and shifters. 

A 10 speed Shimano road shifter needs a 7-9 speed mountain/road rear derailleur or a 10 speed road derailleur. A 10 speed Shimano mountain shifter needs a 10 speed mountain shifter. You're proposing a road shifter with a mountain derailleur (both 10 speed) which won't work. 

Once you know the cassette size you want to deal with, you can choose the appropriate model of rear derailleur (long cage, medium cage, short cage, for example) by looking at the required capacity.