Despite the name and some visual similarities, there is a big difference between the desktop version of Windows and Windows Phone. They are two entirely different products when you get down to it, each with its own release schedule. Just because the desktop version of Windows 9 is rumored (Microsoft hasn't officially set a date and even those will sometimes slip) doesn't mean anything about the Windows Phone version.
Whether or not any given device is upgraded would also largely depend on things like carrier approval, whether or not the hardware of current phones will be up to the task of running Windows Phone 9 well and a myriad of miscellaneous issues that will invariably come up. Lower end units will have a much higher chance of getting left behind compared to higher end units, and first party (like how even before Microsoft officially owned Nokia's mobile business, it was an open secret that Nokia was the Microsoft preferred brand) phones tend to get updated more often, but with cell phones there's never really any guarantee. Some phones never see a single update to the software whereas others will continue to get upgrades 2 years after they were released. However, as a general rule, cell phones are typically on a 1-year cycle. Meaning every year, the current flagship device is replaced with a new flagship device. After a device is no longer the latest flagship, it quite often is subject to a rather swift death by the end of the second year of its life.
How likely is it that current Windows 8 phones (Nokia Lumia for example) will be updatable to windows 9 when it comes out next year? Looking at getting a new smartphone, but not sure whether to wait until windows 9 phones come out.

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