Thank you for contacting us.
I tried to reproduce the issue that you are describing but I couldn't. What I did was, I opened the Examples solution in Blend, then the FirstLook.xaml, right-clicked on the UserName TextBox, then EditTemplate -> Edit a Copy and Blend created a new Template for the TextBox, I modified it and it was correctly applied. Is this case not working on your side, or you are doing something differently? You can also try to add just a RadTextBox to a new project and edit the original template and it should work.
I hope this information helps. Let us know if you are still experiencing difficulties.
All the best,
the Telerik team
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