Aerobat
As has been posted here numerous times, using a "derate"is EXACTLY like installing a different engine on the airframe. All performance is now computed at the derated thrust, this is quite different than with "reduced" thrust settings (assumed temp, for example). Here all the performance calcs are based on the rated power (maximum or the derate selected), so it is perfectly valid to add power from the reduced setting to the rated power without concern about Vmcg or Vmca. Dynamics of adding thrust need to be accounted for, snappy power changes maybe deatablizimg even if the actual power vs. speeds is acceptable in a stable situation.
GF