We are planning to migrate to migrate our Oracle 11g to Postgres 11.2 Database and along with that Jira Version from 6.4.13 to 8.20. ( Currently we are on Jira 6.4.13 & Oracle DB 11g )
However we are having some difficulties when moving to version 7 and 8 with the behavior scripts and Jira Licensing as we run on Jira Core License.
Due to the time crunch of moving to Postgres , we are planning to run Jira 6.4.13 on Postgres 11.2 database. I understand that it is unsupported but if the testing goes well on UAT environment , is it safe to move ahead on production ?
So to sum up , we will be just switching databases on Jira 6.4.13 from Oracle 11g to Postgres 11.2
I would not recommend it. You'd have to test this on UAT thoroughly to make sure Jira can run without issues.
What I would do in your case is install a postgres 9.3 instance just for Jira and move to postgres 11.2 whenever you are ready to upgrade to Jira 8