If that subscription moves to the premium group C, then they would move to that group during the trial period also
They will indeed remain in that group
The group change action occurs upon point of having that subscription, whether they are on trial or not. Trial has exactly the same benefits as when they begin to pay
If they do not pay for the subscription after their trial period, they will revert to their previous group once it expires.
If they were already a member, it would revert to their previous group. As you are essentially registering in order to purchase the item, they would not have been a guest, their last state will have been whatever your first members group is on registration (by default thats 'members')