Hello,
today i have try in a test environment to upgrade a JIRA 6.2.4. Version directly to JIRA 7.3.x
I get the effect that JIRA starts clear and all Data is available.
But when i restart JIRA after this migration, JIRA won't start again, it looks like the data is not consistent.
The Upgrade self i try it with out the Migration of the Attachments because to sync more then 200GBs will take to much time.
So, it is better to migrate from 6.4. to 7.0. to 7.3.1 to solve this, or ist this a Problem that i don't have used the attachments and the Upgrade task was not finished clearly?
Maybe someone has make the same "mistake". 
Regards
Michael