[2312222]

Well my main unit has these modules:

  Behavior = WeaponSetUpgrade ModuleTag_05
    TriggeredBy = Upgrade_TankGun_HEAT
  End

  Behavior = ObjectCreationUpgrade ModuleTag_06
    UpgradeObject = OCL_TankGun_HEAT
    TriggeredBy   = Upgrade_TankGun_HEAT
  End

  Behavior = ProductionUpdate ModuleTag_07
    MaxQueueEntries = 1; So you can't build multiple upgrades in the same frame
  End

So the upgrade "Upgrade_TankGun_HEAT" switches the tank's weapon to another weapon type. At the same time the dummy drone is created (currently an exact copy of the US Battledrone).

ObjectCreationList OCL_TankGun_HEAT
  CreateObject
    Offset = X:0 Y:0 Z:40
    ObjectNames = TankGun_HEAT_Dummy
    IgnorePrimaryObstacle = Yes
    Disposition = LIKE_EXISTING
    Count = 1
    RequiresLivePlayer = Yes
  End
End

The drone has that module:

  Behavior = UpgradeDie  ModuleTag_12
    ;This frees the object based upgrade for the producer object.
    DeathTypes = ALL
    UpgradeToRemove     = Upgrade_TankGun_HEAT ModuleTag_13
  End

That means whenever the drone is killed, I am just shooting it by an AA-weapon at the moment, the upgrade should be removed and the weapon restored.

Well, the upgrade is removed, but the weapon stays. So that's my problem.