Simple command based off the addon UpRank - Addons - World of Warcraft - CurseForge.
Just type /spellrank and it will replace all spells on your action bars with the highest rank available.
Don't use in combat.
Has only been tried on a shaman lvl 40 ( does not change spells on the special totem bar for shamans ) and a lvl 50 warlock but worked flawlessly.
The Spellrank Team
