Previously, Apple had said that only Macs purchased between June 6 and the arrival of Lion would be eligible for the Mac OS Lion Up-to-Date Program.
Apple normally charges $31.99 for Lion.
On its website, Apple laid out the deal to customers.
Buyers who purchased a new or Apple-refurbished Mac between June 8 and July 20 must complete and submit an online form no later than Aug. 19 to receive a redemption code that, when entered in the Mac App Store, lets them download a free copy of Mac OS X 10.7, aka Lion.
People who buy a Mac after June 20, however, are also eligible for a free upgrade if the machine does not include Lion. In other words, Snow Leopard-powered Macs already in the sales channel—on a store shelf, say, or in a warehouse—also qualify for the program.
Those users must submit their online request for Lion within 30 days of the purchase date, Apple said.
Only Macs purchased through Apple’s online store, at Apple’s brick-and-mortar stores and at authorised Apple resellers are entitled to a free Lion upgrade.
Customers can determine whether the seller is Apple-authorised by entering their postal code or city and state in the field at the far right on this website.
Both Lion and Lion Server are included in the Up-to-Date Program. A free copy of the latter, essentially a $50 add-on for Lion, will be offered to people who purchased a Mac Mini Server or Mac Pro Server.
A complete list of eligible Macs , both new and Apple refurbished models, are available on the company’s website.
According to the deal’s terms, the free copy of Lion can be installed only on the recently-purchased Macs and cannot be used to upgrade other Macs that the customer owns.
“The OS X Lion software content code must be used solely with the products that qualify for this offer,” Apple’s terms stated.
Although some customers reported problems obtaining redemption codes on Thursday, that issue has apparently been fixed.
On Thursday, readers of Cult of Mac said that they were refused a code when their machines’ serial numbers were judged invalid, even though they bought the systems through the Apple online store during the eligible period.
Others reported similar problems on Apple’s support forum.
Apple has posted details of the Up-to-Date Program and instructions for obtaining a redemption code on its website.