How would you introduce this breaking change if you were the maintainer?

Maybe this:

1. In the first release, introduce a config value which enables the new behavior.

2. In the next release, print a warning or refuse to compile if the config option is not set.

3. In the final release, make the new behavior the default.

Plenty of time to adjust your code to the new behavior even if you don't read the changelogs. But it won't work if several releases are skipped during updates.