Once a subscription set is committed, there is no way to cancel it. Even if a new subscription set is committed, the current one needs to finish bootstrapping before the new one can start bootstrapping. Some users would like the new subscriptions set to start bootstrapping immediately, hence cancelling any old subscription sets.

There could be an API call which:

1. Pauses the sync session
2. Marks the old subscription set(s?) Superseded
3. Resumes the sync session