In the above video, before the Galakrond is trasformed, Dragonblight Cultist summons two 1/1 imps.
However when the Galakrond is transformed into a shaman minion(Al’Akir), Dragonblight Cultist triggers shaman Galakrond’s hero power.
It seem quite weird that warlock doesn’t control any shaman Galakrond but invoke card triggers shaman Galakrond’s hero power.
Also if Galakrond transform into neutral minion or non-Galakrond class minion, invoke animation is still pops up when you play invoke cards but it doesn’t trigger any hero power.
Once determined, your main Galakrond remains the same for the rest of the game unless it’s destroyed, transformed, or you replace your Hero with a different version of Galakrond.
https://us.forums.blizzard.com/en/hearthstone/t/discarded-enemy-galakrond-but-still-invokes-are-effectiv/20326
More hilariously:
https://us.forums.blizzard.com/en/hearthstone/t/galakrond-transmogrifier/20809
Per the explanation here, destroying or transforming it should clear it: https://playhearthstone.com/en-us/blog/23190444