I need to unsubscribe 123 members from one plan and add them to a identical plan with just a new expiration date. How do I do this?

Using CBSubs built in Import feature you can move users from 1 plan to another or the same place for instance with different status, dates, etc.. Please review your documentation regarding usage of the import feature.

You need to first disable their subscription then enable it. Please try the below example.

Import users from: All users from a given subscription plan
Source Subscription plan: Your Plan
Status: Active

Destination Subscription plan: Your Plan
Current state: Unsubscribed
Subscription date: Today

Perform a test import instead of a real import of the above first to ensure it'd work! If it works fine and as expected run the below.

Import users from: All users from a given subscription plan
Source Subscription plan: Your Plan
Status: Unsubscribed

Destination Subscription plan: Your Plan
Current state: Active
Subscription date: Today

The best option to migrate users is truly to make a copy of your plan then move them from the old version to the new copy, which is the easiest and safest approach as the above would active a subscription for those who legitimately unsubscribed.

Following Users could not be subscribed (usernames) because either: (A) an exclusive active subscription exists that would conflict with the imported user subscription, or: ( it is a children plan but the parent plan subscription does not exist:

Did you first unsubscribe them? You need to unsubscribe (expired? cancelled?) them from the plan, then re-import them to the same subscription as subscribed (active). Aside from that not much more I can advise. CBSubs has no mass processing of subscriptions like you're wanting. Another option is to just directly edit them from the database to mass change their expiration dates, quite easy if all of their expiration dates are going to be the same.