auto upgrade

I am trying to design a feature which will automatically promote a registered user to another plan after a specific period of time (as a 'good conduct prize'). Thus extra content (in this case) is automatically made available to the user if they stay registered. How can/ can this be achieved in CBSubs 1.0.3?

I believe the only way to achieve this is to insert a subscription for the user into the database with a start date in the future. This can be done with CBSubs SQL Actions or CBSubs URL integration. Examine the subscriptions table in your database for help on structuring the insert. There are multiple tables you'll need to insert into such as payments, baskets, etc...

I imagine another way would be to hide the plan and then use an autoresponder to mail the plan url at the appointed time. Is the CBSubs mailer plug capable of this or is it fixed at the point mailings are sent?

CBSubs email options are at fixed points, specifically plan activation, renewal, expiration, etc... They do not send at specific times or intervals.