The first thing to be aware of is that ASP and ASP.NET applications can be run on the same server at the same time with no problems. Different file extensions, configuration models, and processing engines mean that there are no conflicts between them. An application can even have parts that are written in ASP and parts that are written in ASP.NET.
This coexistence can be used in several ways to give you flexibility in converting ASP applications. The following is a list of several different upgrade paths:
Although upgrading the entire application at once is probably the most elegant solution, doing so might not always be possible. Large, complex sites might need to be rewritten one segment at a time because it's necessary to balance upgrading with keeping the site available and adding new features.