How do you port projects and issues from one Jira server to another ? To complicate things, the source server is 6 and target is 7.
If official guide does not suite your needs, you have other option.
If you or your developers familiar with SQL/ETL you can migrate and transform your data from one database to another. Of course, you should (partially) follow official guide in this case too.
P.S. I completed migration from JIRA to Redmine 3 years ago. It is complex, but possible.