I'm not sure you're getting it here. You cannot undo this. Older versions of MySQL don't support utf8mb4 properly. You cannot do hand-wavy things to add that support. They simply cannot support those character sets. Trying to "fix" an export file will simply result in the data becoming corrupted if you use any of those unsupported characters.
If you're going to be importing into an older version of MySQL, then you need to develop on that same older version of MySQL. Downgrade your local dev version if you need to do this sort of thing.
If you want to try to search and replace "utf8mb4_unicode_ci" with "utf8_unicode_ci" in your export file, then that might work. Probably won't though. And it will definitely result in data loss most of the time.