Spellcasting with 2H weapons?

[Purvis Anathema]

Can you hold a 2H (not 1+, actual 2H)weapon and cast spells that have the Somatic component?

The text of somatic components seems to imply you can:

CRB wrote:

A somatic component is a specific hand movement or gesture that generates a magical nexus. The spell gains the manipulate trait and requires you to make gestures. You can use this component while holding something in your hand, but not if you are restrained or otherwise unable to gesture freely.

but the Interact action seems to imply that any changing of hand placement is an Action.

So do I cast while holding my polearm normally? Do I have to pay an Action to take my hand off the weapon? How about to put it back on the weapon?

Do we have a definitive answer for this? I tried searching but failed miserably.

[Voss]

Somatic is fine. Material explicitly requires a free hand.

[Purvis Anathema]

Voss wrote:

Somatic is fine. Material explicitly requires a free hand.

So the somatic component does not require me to change my grip. But if I did something else that required that I use that hand (like a Material component), then I would have to pay one Action to replace the hand.

Is that right?

[Voss]

Right.
A free action to drop the hand
whatever actions to cast the spell
an action to put the hand back.

[rainzax]

Voss has it.

To your other question "Is it an action to release grip or to regrip?" the answer is Release (Free Action) and Interact (Single Action) respectively:

Check source CRB Table 6-2 to see "Change your grip by removing a hand from an item" called a Release action, and "Change your grip by adding a hand to an item" called an Interact action.

Interact

Spoiler:

Single Action
Manipulate
Source Core Rulebook pg. 470
You use your hand or hands to manipulate an object or the terrain. You can grab an unattended or stored object, open a door, or produce some similar effect. You might have to attempt a skill check to determine if your Interact action was successful.

Release

Spoiler:

Free Action
Manipulate
Source Core Rulebook pg. 470
You release something you’re holding in your hand or hands. This might mean dropping an item, removing one hand from your weapon while continuing to hold it in another hand, releasing a rope suspending a chandelier, or performing a similar action. Unlike most manipulate actions, Release does not trigger reactions that can be triggered by actions with the manipulate trait (such as Attack of Opportunity).
If you want to prepare to Release something outside of your turn, use the Ready activity.