First, I made a backup for all items assigned old templates. Then, I created new templates and all items are assigned with the new templates.
After assigning new templates, there was something problem and I moved back to all items assigned old templates.
Then, I tried to remove the new templates, but Sitecore say that "The template is used by at least one item".
I can assume and check all moved back items and no items are using the new templates.
What is the issue??