To avoid this, you will need have a clear policy on what you're willing to support (eg versions up to 1 year old, or "major version - 1", etc.), and stick to it. Otherwise, people will expect you to support all of it, forever.
If you want people to keep up to date with your releases, you either have to provide an automatic update facility that they have to manually turn off (ugh, too hard), or you have to dangle desirable features as carrots in front of them, so that upgrading is exciting instead of a social obligation.
Include the currently running version in every logfile, if not every log statement. Make it clear to you where they are and run your own run-behind that upgrades only after they do. This will be painful, but it will save you overall time.
Dealing with the long tail of customers waiting to upgrade will be a persistent challenge though