Hey there, llorenç.
I would personally recommend you to perform the incremental upgrade of your instance as per the following:
To make sure that the upgrade process goes smoothly, please ensure that you go through the following documentation that will help you to understand the steps better:
Also, please make sure that you run through the JIRA Release Notes and Upgrade Notes to familiarize yourself with the changes in the UI and features that are being introduced in JIRA 6.0:
Hope this helps and good luck!
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