Some useful links.
Regarding to the upgrade, this is the main source you can use for upgrading your instance:
To check the add-ons availability you can go to the https://marketplace.atlassian.com/ to see if they will be available in the newer JIRA version.
To check the release notes for the intermediate versions and see what have changed from the old version until now:
To check the application links compatibility in case you have other products talking to JIRA, you can use the following document:
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