Plan upgrade - non-exclusive

Joomla 3.8.6
CB 2.1.4
CB Subscriptions 4.2.0+build.2018.04.03.17.01.22.ef7c42874

The site involved has a series of plans for site access. A parent plan (Subscribing) (non-exclusive) with child plans (e.g. Student, ... (some exclusive, some not)).

Completely separate the site offers online courses. These courses' parent plan is 'Top' and all the course plans are / will be non-exclusive. A user is NOT required to have a Subscribing plan to register for online courses.

The problem occurs when an existing user has a Subscribing parent plan whose child plan (e.g. Student) is expired (NOTE the Subscribing parent plan is non-exclusive and does never expires.) When this user with the expired child plan tries to register for an online course, they are being forced to update their Subscribing plan (which should not be a requirement). It appears that the 'Upgrade' in CB Subscriptions, applies to all existing user plans and not just the plan being registered for in this instance.

Is there a way to set up two different 'paths' of plans so that upgrading or registering for one is in no way constrained by the status of any other path (other parent)?

Current plan configuration
We want users to be able to register for an online course even if they have an expired Student subscription with an active Subscribing subscription.

The registration works fine if the user has no existing Subscribing plan or if they have an active Subscribing and child (e.g. Student) plan. It does not work if their child (e.g. Student) plan has expired.

Thanks for any suggestions or insights.

Not sure I understand. You are protecting content with your Student plan and you still want them to have access even if they've expired? Just remove that protection otherwise I don't understand what you're asking as sounds like it's working as intended.

The Student plan expires after a set duration (1 year). The Subscribing parent of the Student plan doesn't expire, but only provides them access to their profile so they can renew their expired Student plan. The content a student has access to does not include online courses. Online courses have a separate fee, thus the separate plan. Each course will have its own plan (price) and we want users to have access to that course for a defined period (1 year currently) so they can access their certificate from completing the online course.

So online course subscribers only get access to that course material and their profile. They do not get the site access that a Student would. It might help to note that 'Student' and online courses are completely unrelated contexts.

As previously mentioned if a user has an expired Student plan (which they could renew since they have access through their parent Subscribing plan), but they only want to register for an online course, the message we get from CB Subscriptions is "Chosen plans combination is not allowed (you must choose coherent plans selection, e.g. mandatory subscription(s) must be active or mandatory plan(s) must be chosen). "Subscribing" has mandatory options "Student', ...". But the user doesn't necessarily WANT to renew their Student plan. They just want to register for (subscribe) to the online course plan. But CB Subscriptions will not allow the registration to the online course plan because it knows their Student plan has expired.

What this tells me is the the CB Subscriptions 'upgrade' is incapable of understanding that the user wants to only register for a completely separate plan and NOT renew an expired plan.

Ok, I see the issue. Your plan setup is forcing them to be active Student or Professional subscriber. Those are your only 2 exclusive plans. Exclusive in CBSubs also means mandatory. Meaning 1 of those is mandatory. As I understand it you have the following.

Student (Exclusive)
Professional (Exclusive)
- Other Plans (Non-Exclusive)
Courses (Non-Exclusive)

Adding a top level exclusive free action would also help, which would be fore users who only want the courses.

Free (Exclusive)
Student (Exclusive)
Professional (Exclusive)
- Other Plans (Non-Exclusive)
Courses (Non-Exclusive)

If the above isn't correct please provide a better structured view of your plan setup as it's hard to understand how they're reaching Student and Professional from your above post.