> I would NOT warn the end users about needing to upgrade

People using your product are discussing it on review sites, on social media, with friends/family, etc.. If you silently give some of them a broken product, a few will contact support; but I imagine most will assume your product is just low quality. So that's the word they'll spread.

For corporate users: well, you'd need to talk with your customers. They might be facing a seriously expensive upgrade process, and could be extremely unhappy to hear you're setting an arbitrary deadline for them.