It's technically a merger. It's the next version of .NET Core after v3.0 but jumping ahead to v5.0 and dropping the "Core" designation, and then merging in part

It's technically a merger. It's the next version of .NET Core after v3.0 but jumping ahead to v5.0 and dropping the "Core" designation, and then merging in parts of Mono, AOT toolchain, and even more of the (upgrade) bits of the classic framework. You might be able to get pretty far with just a framework switch. The RC is already out so try it. You'll probably have to update the csproj/sln files at a minimum and fix some known API differences but it's unlikely you need an entire rewrite.