Microsoft's ambitious plan to get Windows 10 running on a billion devices within the next few years depends to a large extent on the success of its free upgrade offer.

When the company first announced the terms of that offer last May, it literally included an asterisk and fine print. Those terms have changed slightly over the intervening months, but one element has remained constant: The offer is good for one year after the availability of Windows 10.

Here's the actual wording of the offer, as it appears today:

It's free and easy

Upgrade confidently - 100+ million fans have upgraded and are loving it. You'll have a free, full version of Windows 10 -- not a trial or a lite version -- if you complete your upgrade before July 29, 2016.

And this is what currently appears in the fine print at the bottom of that page (emphasis added):

In fact, Microsoft's real goal with this upgrade offer isn't just to get its installed Windows 10 base to a billion. The long-term goal is to help close the books on Windows 7 in an orderly fashion before its extended support commitment ends on January 14, 2020.

Some of those Windows 7 PCs will simply be retired, of course. But what about those that are only a few years old and have more than four years of usable life ahead of them? For Microsoft executives, the prospect that hundreds of millions of PCs will still be running Windows 7 on New Year's Day 2020 has to bring back unpleasant flashbacks of Windows XP's messy end.

Ed Bott sees at least three possible scenarios playing out when July 29, 2016 rolls around...

Read more: What happens to those free Windows 10 upgrades after July 29, 2016? | ZDNet