I changed my transmition fluid yesterday, and it was pretty metalic. It makes no noise. shifts fine. Runs great.

maybe flush it well then refill and go by that great motto of
if it aint broke dont fix it,
 that applys really well when auto trannys are involved.

Bearing wear will cause the fluid to get metallic.  I wouldn't worry about it until something gets real noisy.  Even then you wouldn't need a whole new transmission.  I know if mine starts acting up they seam simple enough that I will tear it apart and replace bearings/seals as necessary.