It won't upgrade, there will be different versions on your server. 1.143 is integrated in Windows 2003 (Perphaps will change in Windows 2003 rc2)
As jbuts mentioned, 7.5.6 will support ASP.NET 2.0, also it will support more then one application pool per domain (ASP.NET 1.143 and ASP.NET2.0)
John S.G.