In my test, the schedule was regenerated from the available version. I was using a 7.3.1 system.
I created an agent in ruleset 01-01-01 with a recurring schedule. I then saved the agent rule into 02-01-01 with a different recurring schedule. When the initial schedule was created, it used the 02-01-01 version since they were both available at that time. Then I changed my 02-01-01 version to Not Available and deleted the agent schedule. When it regenerated, it had the schedule from the 01-01-01 version, which was Available.