TOPIC:

If you are on JEM 2.3.5 for Joomla!3
Before you migrate to JEM-for-Joomla!4
Wait! There will come a step-by-step tutorial to migrate: (it's better to know just now!)
(Make a backup!)
Update to Joomla! 3.10.11
Update to JEM 2.3.6 (Premigration version)
Make a new backup!
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Disable all JEM extensions, component, all jem plugins, all JEM modules. DON'T uninstall!
(During upgrade process, it's suggested to switch to Joomla3 default template "Protostar")
To prepare the update from Joomla3 to Joomla4, go to Components > Joomla! Updates, choose Options and in Update Channel choose
Joomla Next.
Save.
You'll get now the informations about which other components are not ready to migrate. Perhaps you have to disable/uninstall other
components/modules/plugins not prepared for Joomla4)
Update to php 8+
Then upgrade from Joomla3 to Joomla4.
Install JEM 4
Before switching to the frontend, change the positions of the JEM-modules and the main menue module, to the positions of your new Joomla4
template (e.g. the Joomla4 default template "cassiopeia"!
If you get a red window, go to configurration>site>SEO and disable URL rewrite (htaccess)
Enable all JEM 4 (component, modules, plugins)
Adapt the JEM settings (there are changes: e.g. datetime format)
Check everything, if it works!
Finish

IMPORTANT:
If you have JEM 2.3.6 or higher, you don't need to uninstall JEM. DON'T!.
Only Unpublish/Disable all JEM plugins and all JEM modules.
Install JEM 4.0 rc1 or higher as an update extension.
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If you have overrides within JEM, you must think for that, when they are still there without changings, they can create problems!