I believe that you'll have no issues after upgrading the SQL server. However, just to play safe, I'd suggest to create a trial environment to test it.
Therefore, download a new JIRA > Install it as a evaluation license > Install a SQL server (express edition) with SP3 and import your valid JIRA XML backup. It should works.
Anyway, you always have https://support.atlassian.com for emergency. Just don't forget to keep a valid database and JIRA XML backup in a safe place.
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