I don't have 2007 so I'm at a loss but this blog article seems to address it. They had trouble even after uninstalling the addin because it remains checked in the dialog but you just want to uninstall it and overwrite it so it may be just what you need:
[vba]Microsoft.Office.Interop.Excel.AddIn addin = Globals.ThisAddIn.Application.AddIns.Add(fullPathToAddin, false);
addin.Installed = true;[/vba]
change installed = true to false.
http://social.msdn.microsoft.com/For...b-74211cda7299