I'm sure people will upgrade willy-nilly and still call you names, but at least they've had plenty of warning.
Besides, if your program doesn't work on their test environment after updating because it needs some extra config to enable legacy behaviour, their update process will have to slow down (alter config and retry) but no serious downtime will occur.
Bold of you to assume they have one of those! Personally I assume they will break production, then call you names because of it.
For open source software: too bad. The AS IS part of the license should cover expectations well.
For commercial software: depends on the contract, really. If the contract hasn't specified anything, an upgrade document will work fine in many cases.
For enterprise software: your customers won't update your software anyway. I'd go with warnings just to be safe. Maybe do it in three stages, first warnings, then severe warnings, then errors.
If you really want to grab attention for your warnings, prefix them with <<COMPLIANCY RISK ALERT>> and you'll be sure to get them to read the upgrade guide!