Upon trying to install the export file the UI will display the following:

The file you are trying to import is from JIRA 7.0.0-OD-01-193, which is newer than your current version of JIRA and will not work. The import file claims it can be successfully imported into JIRA 6.4.8 or later.
If JIRA 6.4.8 is not yet available for download, please contact support.

The log file will indicate:

Downgrade cannot proceed: Cannot downgrade data - missing downgrade task 70007