If you have a commercial/academic license for the server products, you can generate a developer license valid for releases of JIRA that were made while your license was still under active maintenance, you can learn about that in this Licensing FAQ.
If you are planning to upgrade to version of JIRA released after your software maintenance expired, you can use a fully functional evaluation license. Then when you purchase your renewal, or new license, you can just copy and past the license into the JIRA instance.
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