Can a character who casts Shapechange and turns into a spellcaster use innate spellcasting to cast spells with a long casting time?
A specific example is when a druid casts Shapechange to turn into a Planetar that has innate spellcasting. They then cast the Commune spell that Planetars can cast 1/day. The problem is, Commune has a casting time of 1 minute, and Shapechange is a concentration spell.
On pg. 202 of the PHB under "Longer Casting Times", it states:
When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so.
I'm wondering if the spellcasting being innate changes that in some way.
Best Answer
Attempting to cast commune will end shapechange
As you noted, shapechange is a concentration spell which means that concentrating on another spell or casting a spell with a long cast time will end it (see also this Q/A).
The Planetar's Innate Spellcasting only says that (emphasis mine):
So the only special property of the Planetar's Innate spells is that they require no material components. All other aspects of the spellcasting function as normal, as per the rules on innate spellcasting, including the concentration requirement for casting a spell with a long cast time that you've already quoted.
Therefore, casting commune as a Planetar requires concentration for the duration, which means that attempting to do so would end shapechange as soon as you started casting commune.