Upgrades between modern .NET (Core) versions are simple and painless. It’s just a single-character change in the .csproj file, plus maybe upgrading some dependencies (but old versions will usually work).
Upgrading from the classic .NET Framework to the modern .NET is usually a long process. Simple console apps, desktop apps, and libraries might be doable with an automated csproj upgrade. But Web apps require a rewrite from scratch, because the old ASP.NET is incompatible with the modern ASP.NET Core.
Upgrading from the classic .NET Framework to the modern .NET is usually a long process. Simple console apps, desktop apps, and libraries might be doable with an automated csproj upgrade. But Web apps require a rewrite from scratch, because the old ASP.NET is incompatible with the modern ASP.NET Core.