On celexa 20mg for years for depression. dr. said to d/c that & start cymbalta (duloxetine) 30mg for depr. & fibromyalgia. do i d/c 1 & start the other all @ once?

No: I wouldn't. they are different drugs with different mechanisms of action, which may lead to withdrawl. If you are going to stop celexa, I would taper over at least one month while on cymbalta (duloxetine).

It depends: Different physicians switch antidepressants in different ways, and I would call your physician to find out how he/she wants to switch.