SolidWorks is backward compatible so you do not really need to do anything to keep your add-in compatible with newer versions. There sometimes may be slight regressions when something needs to be fixed but in most cases there is no problem.
Sometimes you may want to use new functionality introduced in newer versions (or there is a better method of what you did before). In this case you need to replace your referenced Interops/Type Libraries and you can use new API. And, 'What's New' section is a good place to get introduced to a new APIs.
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