With Microsoft cutting the support for Windows 2003 by July 2015, many legacy web applications built with .NET Framework 1.1 have to be migrated to a newer Windows Server 2012 R2. The dilemma is that .NET Framework 1.1 is no longer supported (it’s not even possible to manually install the .NET Framework 1.1 on these platforms) on Windows Server 2012 or Windows Server 2012 R2.
The proposed solution by Microsoft is to install the .NET Framework 3.5 SP1 which includes the .NET Framework 2.0. Most ASP.NET 1.1 applications should run under .NET Framework 2.0 without re-compiling or re-writing of codes so long as they don’t have any runtime breaking changes, whether it’s API or behavior.
Further Reading
Breaking Changes in .NET Framework 2.0
List of Breaking Changes
Running .NET Framework 1.1 Apps on Windows 8, Windows 8.1 or Windows 10
Microsoft .NET Framework 1.1 and 2.0 Compatibility
Runtime Breaking Changes
