Move existing subscription items between subscription groups

I do not know if you can move an item from one group into the other.

But here is another solution:

Maintain two groups.

Group A already has item 1 with duraction 1

Create a new IAP with duration 2 and add it to Group A

Group B has item 2 with duration 2

Create a new IAP with duration 1 and add it to Group B

In the updated app, offer only Group A to anyone who does not already have a Group B subscription.

If someone already has a Group B subscription, offer only Group B subscriptions.

Sounds like you need to leave group A and group B alone, and create group C. That way, you can continue to offer only group B subscriptions to some that already has them.

By adding an item to Group A and also adding an item to Group B you allow all current subscribers to upgrade and downgrade from their current subscriptions.

Is it like me having two keys for same subscription?

Yes, you would have two IAPs for the same subscription; 4 IAPs in all. Therse would be in two different groups and every user, in one or the other group, could upgrade and downgrade within their group. Unfortunately, you need to maintain two groups because you alreaday have two groups.

Ok I will have to do as you suggested.