Byron Lawrence , Sep 27, 2005; 11:19 p.m.
first thing ideally.
later if you have to. it is possible, as an arduous way around redoing your work and also if you have your work done with layers. to apply noise reduction to your work and copy paste your new backgroung image.
from what I have read several noise reduction softwares use a blank shot to establish a basic noise pattern then use this to reduce noise for later images. I would think applying the effect to processed images would only reduce the effectiveness of the program. this is a guess on my part since I use PS for all my stuff right now. maybe someday I will get some third party plugins.