Is it safe for an application using .NET 4.0 if the framework is replaced with .NET 4.5?-Collection of common programming errors

Check out the list of changes from .Net 4.0 to 4.5 and this blog post for information about compatibility between 4.0 and 4.5.

As long as your project targets the 4.0 runtime you’ll probably be OK, I’ve only experienced one hiccup and that was back in beta (dealing with compiled Regex assemblies). Also, I wouldn’t expect the performance characteristics to be consistent between the devs running 4.0 and those running 4.5.