Comment by theryan

Having just done that on a massive monolith (10y+ codebase continuously developed) I would recommend biting the bullet and doing upgrading all of your projects at once to either .NET Standard or .NET 9 where applicable. Pause development of new features or just do minimal changes on the main branch. We had a ton of footguns (EF6 -> EF Core, WPF, WCF, file serialization, multiple web services) and we were able to complete the migration in 6 months.