A trickier question is, "When will it be worthwhile to make the effort to migrate an existing application from ASP to ASP.NET?" The reality is that while classic ASP and ASP.NET have many common features, for most applications, it will not be a trivial task to migrate an application from one to the other. Changes in languages, as well as some changes in the way that ASP.NET operates compared to classic ASP, mean that depending on how your classic ASP application is structured, migration could require a significant amount of effort.
How do you decide whether a migration is worthwhile? If your application is in production, meets your needs functionally and in terms of performance and scalability, and you do not anticipate further development on the application, it's probably best to simply run it as a classic ASP application. One big plus of the ASP.NET architecture is that it runs side by side with classic ASP, so you don't have to migrate applications. Keep in mind, however, that while classic ASP and ASP.NET applications can run side by side, even in the same directory, they do not share Session and Application context. Thus, you will need to devise your own means of transferring any information you store in the Session or Application collections to and from ASP and ASP.NET, if you want to share that information between classic ASP and ASP.NET pages.
If your application is due for a new development cycle or revision, it's worth examining the types of functionality that your application uses and examining whether ASP.NET would be helpful in meeting the needs of the application. For example, if you have an application that struggles to meet your needs in terms of performance and scalability, the improved performance of the compiled-code model of ASP.NET and its new out-of-process Session State support may enable you to meet these goals easily.
What's important to consider is balancing the cost of migration against the benefits offered by migration. In this book, we will discuss the improvements and benefits offered by ASP.NET. It is left as an exercise for the reader to weigh these improvements against one another and determine whether to migrate a particular application.