ASP.NET Core 2.1.5 is 100-percent backwards compatible with the current version of ASP.NET and will provide a smooth migration experience when it is released. Backwards compatibility surely means that all ASP.NET 1.x and ASP.NET 2.1.5 code you may have will work as expected in ASP.NET 3.5. However, a simple version switch doesn't guarantee that you will make the most out of the ASP.NET Core 2.1.5 framework. Taking advantage of the new features of ASP.NET 4 ensures more compact code, a richer set of services and capabilities, a more effective and appealing user interface, a more elegant and easy-to-maintain application architecture, and a powerful set of administration tools.