I am not sure with Mac but it may be similar. If you loaded your 2018 project into 2019 it will prompt to convert it. Any modification you made can't be converted back to 2018. That's the official word however there is a thread that mentions a reverse method not recommended by Techsmith. You may lose modifications or just break the project.
However : Look for files with this end in the name - _backup.tscproj Before 2019 loads your 2018 project it prompted you as it has to convert it but it also makes a backup with the line above placed at the end of the file name.