Migrating your ASP.NET 1.1 applications to the newer version of the framework is a topic which is under discussion at many places these days, the main reason for this move is because Visual Studio 2010 (the latest VS release to date) has stopped supporting .NET Framework 1.1 applications and it only allows you to have application built on 2.0+ i.e. (2.0; 3.0; 3.5; 4.0). So if you are looking forward to scale your apps, take them to 64 bit, use them with latest Server OS (2008 R2) or want to have (SQL 2008 R2) as backend then the only solution with which you are left out is to MIGRATE.
One can take two approaches to migration, in the first phase you can simply take your app to the latest platform and run it properly (even this is going to take some efforts) and once you are done then revamping the app phase starts and you can utilize the latest available tools and features in order to increase performance and productivity.
Here on I will list an outline which needs to be followed in order to ensure the smooth migration, the attached presentation and the references provided in this article will further explain every single step:
1) Set your migration agenda and goal
2) Develop a complete understanding of the .NET Framework, what has changed, what has been added etc
3) Develop an understanding of ASP.NET, learn, what modifications have been made to the web framework in its version 2.0 & 4.0
4) Prepare and execute a checklist before migration (clean code, 0 compilation errors, rights and privileges etc)
5) Understand and execute the migration process (facilitated by the built-in migration tool & CWA (Convert to Web Application) command).
6) Sort and Fix the errors / warnings faced during the migration
7) Perform post migration checks (32 bit dependencies, incase you have moved to 64 bit; XHTML compliance, incase you are targeting newer browser version; 3rd party add-ons etc)
The above outline is well explained in my presentation (http://cid-866cd84756fa9077.office.live.com/view.aspx/Public/ASP.NET%20Migration.ppsx) prepared by seeking help from the following references.