Updating very old templates

Hello

I use for a long time flexicontent, and even after updating from J1.xxx to J.3.10, with the flexicontent process, I have deprecated lignes and notices.
So I think something went wrong with installation process.

Now I would like to update all my templates. What is the best way to proceed ?
I've seen  www.flexicontent.org/documentation/tutor...late-to-joomla4.html , but I think this is not enough for me, as I have old templates and files from 2009 sometimes. 
I wanted to signal a false information, as in my item.php from 2023 grid template, it's not 
So, so update all my news templates and keep configuration, what is the best way to do ? Is it even possible or do I have to delete and recreate everything ?

Thanks.

 

In fact, the proper way would be to create a new template. I can duplicate category configurations, but is there a way to affect only the new template to multiple cat ?
 

Sorry i dont understand Can you explain more please ?

Since I have been using Flexicontent, the templates have changed a lot. I have made the necessary updates from your tutorials, and it has always worked. But the updates seem to be only for compatibility with the new joomla, without adding the new features. I compared one of my old grid based templates with the 2023 grid.
In my old template: in the display settings, I have only one page, no item-tab for example.
So to change, I thought it was possible to use a kind of basic parameters memorization, and a simple way to modify the template, but finally, I will recreate everything and start from scratch.
Hence my question: is there a simple way to redefine the template en masse, and not go back to each article or category?
Thanks

Hello flexicontent doesnt Lost any feature we just simplify code for maintien it item tabbed already existe
About template assignation if you assign template to type and use défaut in item all bé ok
If you override value in each item no easy way sorry

that's not what I can see.
For example : I don't have seo on old templates. That's something new.
If I just do what you recommand for beeing compatible to joomla 4, I won't get this new feature on my old templates.
But do understand me : that's logical and that is not a problem for me. I know that old template won't be upated, and that because they are custom templates. So I have to make choices, that's ok.

But one new feature could be a method to copy old template parameters to new one. And an other feature could be a method to mass affect one template, just as the same method to copy templates configuration.

Thx