I got an answer for you on this...
Since this product only has regular warranty (not LLW), the available software version goes back to what was available within the 90 days of the customer warranty start date (8/8/2014) on the serial #. That is why when your contract expired, you lost access to software newer than 15.6.1 (released on 10/6/2014). If a product has LLW warranty, and a contract is expired, we use the last contract expiry date in place of customer warranty date of the serial #. That explains why your Summits can "see" newer software.
Regarding your question about downgrading - I would say no. Generally speaking, staying current is recommended.
-Drew