I am thinking about changing my stock air filter to a new K&N SU-8001 filter. Will this minor change need to be followed by a re-jetting of my carb or any other
mods to the bike?? The bike is a completely stock 2004
Volusia VL800.  

Most people will tell you, no difference.
My experience is that it makes a tremendous difference in the carb. You are allowing in more air, at all RPMs. Bikes are set to run lean right from the factory.

About 1 1/2 years ago Cruiser tech tips did a complete walk thru on carb tuning.  

HI:   I have a C50T. I know fuel inj. For some unknown reason Suzuki has seen fit to have the Volusia's and the c50's to run rich. If you don't change the exhaust the filter change won't affect much. It may even lean it out a bit. Your pipes won't get black. If you stick your finger in the pipe  it will get very black. This is too much fuel. IF you change the pipes you will have to rejet. To get more information go to the Volusiarider.com site this is a great place for the VL 800 and C50's..Tons of how too's and lots of help. scruffy..
I install a K&N on mine and it went from 42mpg to 50. It leaned out the fuel so my pipes don't get all sutted up. I all so removed the baffels from the exhaust pipes there are 3inside. Sounds GOOOD. good luck have fun riding.