The collMod command will allow setting or confirming the granularity of a time-series collection:

	{ collMod: "ts", timeseries: { granularity: "minutes" } }

The granularity may only change from "seconds" to "minutes" or from "minutes" to "hours", and will update the stored granularity and bucketMaxTimespanSeconds, as well as update the view definition of the time-series collection.