Can I change block style mid-fight?

No. It is tied to the fighter. Change the fighter, not a button in the pause menu.

Is Philly Shell better for counters?

It can be, if you like the shoulder roll and you accept the open lane on the lead side. It is not a stronger high block.

Sturdy lasts longer. It does not make the meter infinite. You still have to mix movement.

The guard is on the fighter card, not a pause-menu toggle. Style changes how the block holds and how the fighter moves.

UFC 6 block styles explained: Balanced, Sturdy, Evasive, and Philly Shell — what each guard gives and what it gives up.