After adding a mixin to a document type the document editor will only show and render the related mixin plugin for new documents

Currently a mixin, like hippostd:taggable, can be added through the document type editor.
For new documents, this mixin will be automatically available as it's added to the document prototype, but for existing documents this (of course) isn't and cannot be done automatically.
This means that for existing documents the new mixin features won't become available in the document editor, until all (or selected) documents have been updated by adding this mixin to their document node. Which requires writing and executing and updater script first.

A possibly easy and intuitive solution would be to start auto-adding a 'missing' mixin on existing documents on first edit of a document.
This would give consistent behavior for the editors, always rendering the relevant mixin plugin.
What might be tricky but important is that documents without (other) changes should be allowed to be discarded without required feedback.