You should have no problem upgrading from v7.1.2 to v7.4.2. Just make sure you back everything up and follow the standard upgrade process. If you were on a v7.0.x version you would have some issues.
From the link below:
Version specific detail
Upgrading from a 7.1 or later version - There are no known issues.
Upgrading from a 7.0.x version - There is a known issue regarding gadgets, and there is a workaround described in the JIRA Software 7.1.x upgrade notes.
Upgrading from a version earlier than 7.0 - Please consult the Migration hub as it contains information on important user management, application access and log in changes. You must upgrade to JIRA Software 7.0 first, before upgrading to JIRA Software 7.4.
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