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Cisco often maintains several software version trains, concurrently. So it's possible you'll see an older IOS version with a more recent release date than a newer IOS version.
Technically, if the platform supports it, you can move up and down versions (same feature set). Legally, to move to newer versions, you need an active maintenance contract. Not sure about moving down, if you don't have a contract.
If you have an IPBase license, the IPBase image has a slightly smaller footprint, but the IPBase license is also supported by the universal imsage. If you have an IPService license, installing IPBase image, I believe will work, but you'll lose IPService features. (Forgoing assumes you've configured the installed license for execution. If you have sufficient flash, you can install multiple images, different versions and/or feature set. You can also configure to boot into different images.)
