Yup, use the below to do that.

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

Plan Expired
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions (custom value 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.

Is there anywhere where one can see an explanation of those [var2], [var3] ... values ?

What variables are available depends on the trigger used so to an extent yes you can see what those are based off the trigger in the triggers documentation below.

www.joomlapolis.com/documentation/279-community-builder/tutorials/18358-using-cb-triggers

We don't have a means of automating documenting triggers yet, but we've some ideas on how we're going to go about doing that so it'll be easier to look this information up.

I would also like to be able to trigger some code if the subscription of a plan has run out (all the users have a plan that runs out by the calendar year) - a little like the CBSubs mailer - how could I do that? It is for sending SMS messages to users who have not paid the plan in time at the begining of the next year/period - so I have to loop through the respective users and then trigger my PHP script that sends SMS messages.

The "Plan Expired" usage above will fire if the plan is no longer active so it will cover that case. As for sending the SMS you can execute PHP using a Code action. There's no functionality like CBSubs Mailer in CB Auto Actions as it is not a scheduler; the best you could do in that regard is the Internal Users trigger then use the Internal Users URL in a CRON task, which would loop every user through an auto action but that won't have any information about the status of their subscription since that information is coming from the onCPayUserStateChange usages.