Transmission is a little sluggish when shifting?

Transmission is a little sluggish when shifting, not bad but noticeable. I purchased truck do not know about service intervals on service for trans. Fluid is extremely dark. Doesn't smell burnt. I have been told that if I change fluid and filter transmission may start slipping. But I feel if I don't change fluid trans will fail. Do you have a recommendation as to whether I should change filter and fluid or leave it alone and hope for the best?