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Hey Andy
Just to clarify regarding S1C and Management version. (maybe not for you but for others)

Smart-1 Cloud is a service. The concept of a service is that the customer do not have control of the service/environment version, they are provided with the latest and greatest security management version, with NO effort and concern what so ever.

So to your question (which I'm glad that is theoretical only) -no, S1C customer cannot decide or request to downgrade. They can work only with the provided latest version.
One aspect the customer can influence, is that upon upgrade of a major version of their environment (e.g. R81.10->R81.20->R82) - when getting the notice about the upgrade window, they can ask to reschedule their environment upgrade window (Upgrades and any imitated ,maintenance is done in off hours anyway) 
Thanks
Tomer