I was struggling with this this morning and came across this thread. I just wanted to confirm that I was able to fix this by spinning up a temporary install of WordPress. I grabbed the old version of ACF from the site I was exporting from. Did an old-style export/import that uses the WordPress importer XML. Then I installed ACF Pro, did the database upgrade, then exported the JSON from the temp install. Imported that into my new site using the ACF JSON importer and all was well.
