It is up to each individual company that makes Windows Mobile devices to decide if they will offer firmware upgrades or not. If a major new release of Windows Mobile comes out, sometimes vendors will choose to provide an upgrade to some existing users, but others times they do not, or cannot because of increased requirements in the new version.
There has never been a phone where it was legally possible to install a different OS entirely (e.g. from Windows Mobile to Android), although hackers have sometimes found ways to bypass security measures and/or install Linux on phones.
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