JEvents has been Joomla! 3.x compatible since the first release. What we advise you do is STOP and backup your site, once that is do you need to update ALL your Joomla! Extensions straight away, this means you are up to date and any Joomla! 2.5.27 extension should be Joomla! 3.x compatibile. However, check first and make a note if it is not compatible, or needs a specific Joomla! 3.x version. If this is the case you may need to remove it prior to upgrading and then add the version after the migration.
Now all your plugins, modules and components are right up to date we can proceed with the Joomla! 3.3.x upgrade. Go to Components -> Joomla! Update and click Options, top right. You should see:
Select the drop down next to the Update server and select Short Term. Click save & close, then go to the Joomla! Extension Manager, click purge cache and find updates. Now go back to the Joomla! Updater and just update! It should be that easy.
Joomla! offical migration documentation can be found here:
https://docs.joomla.org/Why_Migrate
