New features in Lectora 19 are not backwards compatible with Lectora 18. So things like custom button states or event timeline actions will not work when reverting from 19 to 18. But other than that, Adam makes a good point about testing.
Don’t edit Lectora 19 titles in version 18. You can always backup/archive your Lectora 18 content by exporting the title to zip. Be sure to include the Lectora version in your filename for easy identification. You can then open it with Lectora 18 or with future versions of Lectora.