Best thing you can do is upgrade JIRA 6 to 7.1.1 first. Then create an export and import it in JIRA 7.1.1.
If you don't have the exact same version number in both instances you will receive an error and you wont be able to proceed. You could change the versions in the export text files but there is always a chance it will fuck up the JIRA 7.1.1. or not all information gets imported. So i don't recommend it.
I looked it up to be sure
According to this page:
If you plan to upgrade from JIRA and JIRA Agile less than 6.4.x and 6.3.5 respectively, you must upgrade to JIRA Software 7.0 before upgrading to JIRA Software 7.1.
So in your case you should upgrade to version 7.0 first and then to 7.1.1
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