I could swear there is an option to add users to a group and then remove them or alter them when their plan is done and not renewed.

That functionality isn't a plugin, but just a parameter under the Access tab of your plans. If you're doing this through a plugin it would be by using CB Auto Actions and its Usergroups action type.

I can find the add part (in Access) but I have looked three times (and done searches and perused the manual) and I'll be damned if I can find where a user is assigned to different group on expiration.

The usage under the Access tab doesn't let you select more than 1 usergroup and you can't select a usergroup to assign on expiration. The usergroup selected will be given when they become active and removed when they expire automatically. If you need more indepth control you need to use CB Auto Actions.

If there is no way to do that using CBSubs settings (and I am, in fact, losing my mind) then I'm assuming it can be done using AutoActions or similar. Any insight into this would be appreciated.

The functionality exists in both to an extent. If you only need to manage 1 usergroup per plan then using the usage under the Access tab of your plan is recommended.

What I'm attempting is to use CBSubs to change a user to "Registered Member" group and then on expiration change them back to "Registered." Registered Member would be the way event software (such as Event Booking or Matukio) would know that the user can register for the event.

It sounds like you just need to add "Registered Member" group and remove it when appropriate. That's easily done using the parameter under the Access tab of your plan. If you want a more indepth usage you can use the below in CB Auto Actions for more control.

Plan Active
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
1: [var3] Equal To PLAN_ID_HERE
2: [var2] Equal To A

Plan Expired
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
1: [var3] Equal To PLAN_ID_HERE
2: [var2] Not Equal To A

Replace PLAN_ID_HERE with the ID of the plan you want this to act on.

I simply don't know how you all do what you do between dev and answering these forum questions.

Typically my work day consists of spending the morning clearing support. This could take 1hr to 8hrs (the entire day), but on average takes no more than 4hrs (half the day). The rest of the time is dedicated to development. Some days can get pretty busy on support, but since we use GIT for managing all our development it's not an issue to typically stop mid-development, switch back to master branch, fix something, and release the fix. It was A LOT more stressful when we used SVN and couldn't do that, but that was years ago.

The most time consuming support isn't answering forums, but is install reviews. If there's an issue we can't walk someone through to self diagnose and we suspect it could be an issue in CB or one of our products we have to review their install to see what's going wrong. These could easily take the entire day or even week to track down an issue, but sometimes it's just necessary for us to see the issue to help find and resolve the cause.

Do you all have lives beyond CB????

Absolutely and is a big part of why weekends are our downtime.