Consider a field k, the formal
power series field k((x)) in one variable over k, and a derivation D of k((x)) that
maps k into itself. We wish to replace x by another generator y of k((x)) so that Dy
has a particularly simple expression as a function of y. This is accomplished subject
to certain restrictions on the differential field k, some deductions are drawn, and
there are extensions to the analogous problem for power series rings in several
variables.