Unfortunately, there is no field or setting that allows you to control the behavior the way that you are trying to do so. However, in the below, I have provided you with two solutions that should achieve the result that you are looking for.
My recommendation is that you go with the second option: creating a new template for the new items.
Breaking the Presentation Inheritance
You should be able to achieve what you are looking for by simply making and saving an arbitrary change to the value of the
_Renderings field on the existing item(s) before making your changes to the
__Standard Values. This way, the field on the existing item(s) will no longer inherit the
If you have a large number of items, you can do this with a simple script or with Sitecore PowerShell Extensions (SPE), if you have it.
Important: I advise caution when going down this road. From a maintainability perspective, it's typically better to be able too control presentation on multiple items from a single place. As soon as you break this inheritance, you lose that ability.
Creating a New Template for the Presentation of the New Items
An alternative solution that I recommend before you intentionally break presentation inheritance is to create a new template with its own
__Standard Values for the new items. This template can be set to inherit the existing template, which means that its
__Standard Values will initially inherit the presentation of the existing template's
__Standard Values. You can make the modifications that you want to reflect on the new items, which will break the inheritance, as desired.
The benefit of this solution is that the existing items will still inherit the presentation from the original template's
__Standard Values, while the new items will inherit the presentation of the new template's
__Standard Values. This avoids the maintenance issue and can save you on future effort.
Note: If changing the template is a problem, note that you could also do the same thing with a branch template, but you will lose the presentation inheritance for the newly-created items.