Once an in-app purchase item has been attached to a particular Subscription Group, it cannot be changed anymore. When designing your subscription groups, you should be very careful. The only thing you should ask yourself, is whether or not the end-user should be able to cumulate 2 active subscriptions. If the end-user is not supposed to be able to cumulate 2 subscriptions, then, they should be attached to the same subscription group.
In any case, keep you offers structure as simple as possible.
Do not use subscription groups to reflect the geography of your offers, this is already done by the stores themselves. One unique subscription group should be enough in most of the cases.