That was not possible with CBSubs, and is a very useful feature request!
I've implemented it now, and you can upgrade from the nightly just released. You will then find in mailer (but also in promotions and plan settings upgrades) new settings for conditions and existing user subscriptions for auto-renewal (and many other conditions also on baskets).
Please let us know if that works for you, as we will very soon have a new CBSubs stable release coming out this week.
To upgrade to nightly, just go to CB plugins management, install from web tab, and by logging into joomlapolis.com refreshing with the small refresh icon bottom right you should see the new nightly upgrade proposed.
Thanks in advance for your soon feedback