If i shoot something as a zon, while the arrow is in the air, i know you can swap to something with - res and + eelemental damage and it will increase the corresponding damage.
But if i shoot, and swap to dream/holy fire set up from associated aura gear, is the damage added to the projectile even though it was shot prior to the swap?

No

My understanding is that it now works in d2r, so you should be able to dream swap from bow. Really only relevant to dream sorc.