No. A Stacked subscription (of any level) can only be added to a TexAgs Basic, Premium or Varsity subscription.
Yes. Although your current TexAgs subscription must be restarted, you will receive an automatic credit during the check-out process for what is remaining on your current monthly or annual subscription.
Maybe. Please contact customer support for assistance in setting up a Stacked subscription with a non-standard TexAgs subscription.
No. Because a Stacked subscription requires a base TexAgs subscription (Basic, Premium, or Varsity), the term of both subscription must be synced together.
Like all other TexAgs subscriptions, the auto-renewal of Stacked can be canceled anytime in your account settings up until midnight of the day before your renewal.
Not at this time.