Is it possible to upgrade/downgrade subscriptions?

Imagine following scenario:

I buy Adobe Illustrator (AI) as a singel application subscription. Then after 6 months I decide that I want Photoshop (PS) too. I then get Photoshop.

Question 1: Do I have to do a 1 year subscription for PS seperately or can it go along with the AI subscription (6 months left)?

After another month I decide to get all apps and upgrade to Creative Cloud (CC).

Question 2: Will my AI/PS subscription be canceled then? If yes, does the subscription restart (1 Year) or is it only for the left months (here: 5 months)?

Question 3: What happens if I decide to downgrade from CC to only AI again, is downgrading possible?

Best Regards and Thanks in advance

Proxoss