Will posts written with old editor get mangled?

Hiya,

I most certainly can reassure you!

When you open old content in Gutenberg, it will be shown in a Classic block, that is essentially the classic (old) editor inside of Gutenberg. You may then continue to edit your content like you normally would inside of that, or you can choose to convert the content into the new block format.

Please do note that we can’t guarantee that your new block formatted content will work in the classic editor, should you wish to disable Gutenberg, so it’s advisable to keep a backup during testing if possible.

The only problem that i’ve found is if you’re using the code editor Gutenberg removes some whitespace, making it harder to read. The visual editor seems fine. But this little inconvenience does not in any way destroy your pre-existing content, as long as you don’t convert it to blocks (my experience with that was not very good).