1986 ford escort transmission problem.

In Drive the engine revs but wont accelerate more than 25mph. D1, D2, and reverse work as usual. Fluid is dark, but no debris, no burnt smell.
Should I change the fluid & filter? The shop said that would make it worse.

You have probably already solved your problem - but if not: Changing the fluid wont make the trans younger, and your symptoms are those of a worn out set of frictional parts in a clutch pack. The cure is to rebuild that trans I think. Still, changing the fluid wont hurt either.