Phased roll-out. You first introduce a version that still accepts all extant inputs but will actively warn that there are characters that will be removed in a future release.
Then you wait. Then you roll out a version where the new functionality is flipped on by default, but where you still allow to explicitly toggle to the old one. Then you wait some more.
And then - only then - you roll out a release where the old functionality has been removed entirely.
Then you wait. Then you roll out a version where the new functionality is flipped on by default, but where you still allow to explicitly toggle to the old one. Then you wait some more.
And then - only then - you roll out a release where the old functionality has been removed entirely.