With two slated for 2017, the second's designation for the CBB would mark the end of support for 1607. Depending on the timing of the pair of releases next year, 1607 will almost certainly be supported throughout 2017 at a minimum, and probably well be into 2018, perhaps as long as mid-year.
(Assuming releases in the spring and fall of 2017, 1607 wouldn't see support end until the later winter or early spring of 2018.)
As Kleynhans pointed out, that period -- 2017 and the first months of 2018 -- will be migration prime time for enterprises. " is the version for all of next year," he said, referring to the enterprise. "That's the version people will know."
But 1607 won't be the only version to be deployed by companies as they wend their way through the migration job. Because the support lifecycle of a particular edition, say 1607, is shorter than the average start-to-finish migration, some businesses will be required to start with one version, finish with another.
According to a recent survey done by VMware, enterprise IT administrators and managers said it takes an average of 18 months to complete a Windows migration (and a majority of those polled said they expected Windows 10 to take the same amount of time as past upgrades). That means even if a company began deployment in April 2017 with 1607, it wouldn't finish with that same build because it would wrap up around the end of 2018, months after 1607 dropped off support.
How that will work is unclear, as IT has not been forced to upgrade machines already on the newest OS at the same time it also migrated other PCs from an older edition. The last massive enterprise upgrade cycle was Windows 7, whose SP1 was the sole service pack (and so is still supported); by default, then, companies began and finished their move from XP to Windows 7 with SP1. Windows Vista, which admittedly gained little traction in the enterprise, had a pair of service packs; even so, the first received support for more than three years, from February 2008 to July 2011, enough time for a company, had it wanted to do so, to complete a full deployment.
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