It is your duty to make sure _all_ of your users are able to continue using the same software they installed in exactly the same way for the reasonable lifetime of their contract, the package, or underlying system (and that lifetime is measured in years/decades, with the goal of forever where possible. Not months).
You can, if you must, include an update notification, but this absolutely cannot disrupt the user's experience; no popups, do not require action, include an "ignore forever" button. If you have a good product with genuinely good feature improvements, users will voluntarily upgrade to a new package. If they don't, that is why you have a sales team.
Additionally, more broadly, it is not your app's job to handle updates. That is the job of your operating system and its package manager. But I understand that Windows is behind in this regard, so it is acceptable to compromise there.
We go a step further at my company. Any customer is able to request any previous version of their package at any time, and we provide them an Internet download page or overnight ship them a CD free of charge (and now USB too).