I am planning to upgrade to JIRA 6.4.4, can I upgrade to something more than JRE 1.8.0_40+ , what should I be wary of ?
Nothing. JIRA is supported on all versions of 1.8 except 25 and 31 where there are known issues.
And, in fact, we would strongly recommend using Java 1.8.0_40+ or higher, if you can, for any release from JIRA 6.3 on: https://confluence.atlassian.com/display/JIRAKB/Java+8+Improves+JIRA+Performance+Dramatically
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