Hello Bachar Zeidan,
Inside Sitefinity/Admin/cmsAdmin/Users.aspx.cs you can subscribe for manageUsers_UpdatedUser which will be called when you open a user for editing and change his /hers role.
The user's role could be changed from a DropDownList template which is located above the GridView control. You have to find the control and subscribe for SelectedIndexChanged event
var c = ((Telerik.Security.WebControls.ManageUsers.ListContainer)manageUsers.Controls).FindControl(
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