Jimbo,

There are many different schools of thought regarding your questions; here's my take (for what it's worth)...

You can upgrade to a 9 speed R. Derailleur with no problem and adjust the limit screws for your 8 speed cassette; your existing shifters will work fine. You have the choice to use either a road or mountain DR depending on personal preference.

However, once you decide to make the jump to 9 speed you'll have to replace shifters, cassette, chain and either the freehub body, or buy a new 9 speed compatible rear wheel to accommodate the 9 speed cassette.

To be honest, I don't have enough experience with 10 speed systems yet to know whether or not a 10s R. DR will work on an 8 speed system. I'm sure others have tried and can chime in with their thoughts.

A query... what do you have on your bike right now?