If I rebuild my .NET (in an upgraded framework) will I receive build errors warnings in the solution if the types and members are used?
If I receive no errors or warnings does that mean we are free and clear for potential issues related to obsolete/deprecated features?
In general this is true although it is best to look for and follow a migration guide, which MS always publishes for new versions of .NET. For instance, in the .NET 7 announcement you also have a link to an upgrade assistant: announcing-dotnet-7 To some extent this depends a bit on what you are talking about. Going from .NET Framework 4.7.2 to 4.8 is probably quite easy and about a simple as updating the target framework in most cases. But going from Net framework to Net core, or Net Core 3.1 straight to Net 7 might be also more work. And perhaps in general it will be affected by the extent of your dependencies, the trust you have in your tests (or lack thereof), and the risk associated with any bugs appearing in your application.