I deactivated the compat plugin first, forcing the error immediately (btw. in error.php ). Found the respective line in VSC. Correct and get next error.
Later I systematically went through all the to-be-replaced classes: search in VSC and correct one by one.

I've been in a situation similar to yours:
My proposal: Do it yourself!

The problem is that i have no idea which files or what to do or what to change

Perhaps Ciaran will provide us with instructions on this

The problem is that i have no idea which files or what to do or what to change

What to do can be found here: https://manual.joomla.org/migrations/44-50/compat-plugin/#class-aliases
You have to go through all the PHP files of the template. Best use VSC.

Hello dear J51 community,

I also had it rewritten today using the Carmen template with the help of AI, and it now runs under Joomla 6 without the Behaviour – Backward Compatibility plugin. I'm also no longer getting any errors.

Dautrich, thank you very much for your support and advice.

(Working time approx. 1.5 hours)

All the best for 2026 and good health!

Best wishes for 2026.

So what i'm doing wrong? Went to the above mentioned classes and used VSC, Changed f.i. JFactory into \Joomla\CMS\Factory and that didn't worked so i changed the whole list again into Joomla\CMS\Factory. Still getting errors like "syntax error, unexpected fully qualified name "\Joomla\CMS\Form\FormFieldTplh...", expecting identifier."

Used VSC to open folder and find/replace all files.

Okay, that sounds promising! It was just a matter of revising the .php files, right? Did you adjust the files before or after installing a template in Joomla 6? Did you rewrite everything in the code and then deactivate the compatibility plugin, and it worked without any problems? I would modify the workaround so that I can apply it to multiple j51 templates.

No, I did it the hard way first: While on J5 on a local test site, I set Debug mode to on and error reporting to maximum. Then I switched off the J5 compatibility plugin. After reload, the frontend crashed immediately, but error reporting showed the line number. I opened the complete template as a folder in VSC, giving me direct access to the template's files. I corrected this first error and looked around in the respective file for other old Joomla classes, which I replaced as well.
Repeating these steps as long as I ran into issues finally gave me a first J5-native version (with many hidden issues still). Then I systematically searched for the remaining old Joomla classes in the template, searching through all PHP files including template overrides; VSC is very good at searching.

After I had a working, J5-native template version, I did the identical exercise on the two J51 extensions I use (and on one from a different developer). Having done all this, I used the standard Joomla update procedure to update my test site to J6. Success! As a bonus, I tried to disable the J6 compatibility plugin. Success, again!