These days Reduced Thrust / Derated Thrust is a pretty exact science, but the problem that you mention was fairly commonplace in the early days of reduced thrust takeoffs. It was pretty much left to the operator to use appropriate procedures to avoid negating the advantages of the reduced thrust / power takeoff that you allude to.
The first aircraft that I operated using the reduced thrust principal was to either -
(1) Leave the Thrust / Power where it is if an increase would be involved in setting Climb Thrust / Power, OR
(2) Set Cruise Thrust / Power as the initial setting for climb after Takeoff. (I know of no case where reduced Takeoff Thrust / Power was less than that for cruise (but there's always a first time).
Option (2) was finally settled upon as the best solution, with later increase to normal Climb settings at higher levels when R.O.C. had fallen to a nominal value (1000 fpm for the aircraft I speak of).
What I've said here related to jet aircraft, but the essential principals are the same.
Hope that this helps,
Smokey