Apple charged $19.99 last year for the Mountain Lion upgrade, and will probably use that same price for OX 10.9, if only to, as it did with that version, quickly drive adoption among existing Mac owners.
Alongside the release and pricing information will come details about Up-To-Date, a name Apple's used in the past for the program that will let buyers of new Macs equipped with Mountain Lion upgrade to OS X 10.9 free of charge. For the last three years running, Apple announced the deal on WWDC's opening day.
OS X 10.9 will be distributed through Apple's Mac App Store.
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