Microsoft last week promoted Windows 10's November 2015 upgrade to the Current Branch for Business release track, the first time since the operating system's debut it has approved a build for corporate customers.

"The Windows 10 version 1511 feature update (build 10586), released in November 2015, has been officially declared as Current Branch for Business (CBB), ready for organizations to begin deploying broadly," said Michael Niehaus, a director of product marketing, in a post to a company blog Friday.

The upgrade will be released "in the coming weeks" through Windows Update, Windows Update for Business, Windows Server Update Services (WSUS) and other channels, Niehaus added.

Last year, Microsoft overhauled its Windows upgrade strategy with Windows 10. Rather than release upgrades and updates at irregular, infrequent intervals, it announced it would stick to a set cadence composed of several "branches."

Each upgrade would reach consumers first as Current Branch (CB), then four months later the same build -- presumably of higher quality after millions of consumers had tested the upgrade and Microsoft had made changes to the code -- was to be tapped as suitable for company computers and labeled as the latest Current Branch for Business (CBB).

Initially, that four-month stretch between each upgrade -- which translated into three upgrades yearly -- was to be Windows 10's regular tempo. But Microsoft soon began hinting it wouldn't make that schedule by talking up a frequency of two or three times a year. The rhythm may be even slower than that this year, with just one upgrade likely. Microsoft has formally dubbed the mid-summer upgrade to the CB as "Anniversary Update."

The five-month span between the CB and CBB releases of the November 2015 upgrade is another signal that Windows 10's upgrade pace will be slower than Microsoft originally indicated.

Niehaus also spelled out when the CBB release will reach customers' PCs. The arrival will be dependent on the management tool that corporate IT staffs use to wrangle Windows 10. Those who rely on Windows Update for Business (WUB), for instance, will see version 1511 (the November upgrade) hit CBB devices as soon as Microsoft publishes the upgrade.

Microsoft has also demanded that most Windows 10 users upgrade to a newer release at some point or lose patching privileges. For PCs that have been assigned to CBB, that deadline is unknown: According to what Microsoft said previously, devices on the CBB track must be migrated to a specific upgrade within 12 months of its release.

But the current cadence, along with the fact that the original release -- the one launched in July 2015 and officially called 1507 -- doesn't support user-defined delays, could mean customers have only until the next CBB release appears. If Microsoft shipped the Anniversary Update to consumers in, say, July, and again took five months to get that version to the CBB, corporate customers may be required to upgrade again in December, or eight months from now.

Alternately, it could designate the upgrade-or-lose-patching deadline as the release of the CBB upgrade which follows the one for the Anniversary Update's. (Initially, Microsoft said that several CBB builds would be supported simultaneously.) If that's the case, companies may be able to run 1511 until mid-to-late 2017, or as long as 20-24 months.

The muddy schedule, analysts have said, is probably due to Microsoft's finding its feet in the faster development and release process. That has caused the once-regular release pace -- as described by Microsoft last year -- to become messy and clouded with uncertainty.

In other words, much more like the old Windows upgrade calendar.