According to this: https://answers.atlassian.com/questions/32599/is-it-possible-to-upgrade-to-a-jira-version-which-was-within-our-expired-maintenance-period you can upgrade to any version of jira which was released during your maintenance period. If your maintenance is not expired, you can upgrade to the latest version. Otherwise, see this for release dates: http://www.atlassian.com/software/jira/download-archives
Yes, go to your "system information" page in Jira, and it will tell you your licence expiry date. Then compare the list of releases and their dates, and you'll be able to upgrade to the last one released before the date you found in system info.
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