There are a handful of spells which have sub-spells which temporarily replace the main ability. Some of them can still be leveled while the spell is active, by chosing the sub-spell to level it up. But some cannot be leveld up this way. You have to wait for the spell to finish.

Besides this, some sub-spells have only 1 level, so they always show 1 level, no matter what level your spell actually is. So when ALT-clicking them, they say (level 1), which can be misleading, as allies would assume you have the spell on level 1.

Affected spells are:
  • Ancient Apparition - Ice Blast
  • Bane - Nightmare
  • Io - Tether
  • Elder Titan - Astral Spirit
  • Keeper of the Light - Illuminate
  • Keeper of the Light - Illuminate (Spirit Form)
  • Naga Siren - Song of the Siren
  • Phoenix - Icarus Dive
  • Phoenix - Sun Ray
  • Rubick - Telekinesis
  • Timbersaw - Chakram
  • Tusk - Snowball

* Pick any of the named heroes
* Level up their named spells once
* Cast the named spell
* While active, try to level it up further

Result: You cannot level them up while active. Some of them because they only have 1 level, instead of 3/4 like the main spell, while some other have their leveling up disabled.

Expected: Similar to other spells with replacing sub-spells (X Mark the Spot, Unstable Concoction, Replicate, True Form, Infest), these spell should also be levelable while active.