When building web application in Microsoft platform stack, you will have many choices starting from web pages, ASP.NET web forms, ASP.NET MVC, and ASP.NET core. The problem happens when you want to upgrade from one library to the others. This article will discuss about how to upgrade from ASP.NET to the others.
Upgrade Path
From ASP.NET web forms to ASP.NET Core, you should recreate the project and recreate the codes. You should recreate the entire codes. If you want, you can create the hybrid method as mentioned in this article https://docs.microsoft.com/en-us/aspnet/visual-studio/overview/2013/one-aspnet-integrating-aspnet-web-forms-mvc-and-web-api
From ASP.NET web forms to ASP.NET MVC, you should recreate the project and recreate the codes. You should recreate the entire codes. You can read this document https://docs.microsoft.com/en-us/archive/msdn-magazine/2013/march/asp-net-migrating-asp-net-web-forms-to-the-mvc-pattern-with-the-asp-net-web-api
From ASP.NET MVC to ASP.NET Core, you can upgrade by recreate the project and modift the codes. You should modefy some codes. You read the guide to upgrade https://docs.microsoft.com/en-us/aspnet/core/migration/proper-to-2x/?view=aspnetcore-6.0
Some Tips before upgrade
if you are using asp.net web form, consider to keep the asp.net in the latest version. You dont need to migrate to MVC, except you need the performance of the ASp.nET mvc core
If you are using asp.net mvc, consider to upgrade into the asp.net core. You might need consider to upgrade
If you are using asp.net web pages, consider to do hybrid approaches.
Please make sure you have all the codes, including the dependencies, references, and nuget.
Please make sure you use nuget for dependency library. you can restore later.
After restore the nuget, please make sure your application is built successfully. Never upgrade the library before the codes running well
If you have difficulty in build the codes. Edit the csproj to remove the prebuild nuget, you can find in the bottom of the codes
Put your codes in GITHUB or DEVOPS