Grace periods are applied at the time when there is the decision to expire or not to expire a subscription.

So simply change the grace period anytime, and still active subscriptions will immediately be evaluated with the new grace period.

So active subscriptions but that are in old grace period and not in new one will expire next time the evaluation happens.

Already expired subcsriptions will not be re-activated if the new grace period would mean they shouldn't have expired with new grace period.

Evaluation takes place when user is active in frontend, or in mass-expiry depending on your settings. By default it's 50 at a time maximum when visiting CBSubs main admin panel (number of just expired subscriptions during that page load is at bottom).

Hope that makes sense