I am not sure if this is a bug or working as intended.
I did a Dalaran Heist run as Shaman. During the Tavern breaks I chose to reduce a spell cost 3. I chose the Spellstone that upgrades via overload.
Thing is, once the spellstone upgrades the cost resets to 7 on the upgraded spellstone. Thus the cost is only usable before the upgrade. Seems a bit silly, and in my opinion the reduction should carry over into upgrades spellstones.