• void ChangeFlag(str flag, bool value)
• void ChangeActorFlag(int tid, str flag, bool value)

ACSUtils provides an implementation of A_ChangeFlag for ACS. Before using it, you need to add decorate/changeflag.txt from ACSUtils to your mod.

It works by defining two inventory items for every flag. When given, one of them calls A_ChangeFlag(“FLAGNAME”, true), and the other A_ChangeFlag(“FLAGNAME”, false).

ZDoom later added its own ACS function to do the same, SetActorFlag, but it's not available in Zandronum 3.0.