How to change default TagPrefix of AJAX Extensions' Item?

I installed AjaxControlToolkit in my vs2010. I want all ajax control using ajax tagPrefix. I added

<add tagPrefix="ajax" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/><add tagPrefix="ajax" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit"/>

in web.config. now, the UpdatePanel can use 'asp' and 'ajax' tagPrefix. both are woring. but when I draw the control from Toolbox, Default tagPrefix is 'asp', how can I change it to be 'ajax'?