Some options:
If you are copying the entire source code for your library into your new project, you can use a refactoring tool like Resharper to "Adjust Namespaces". This is a pretty quick and safe refactoring.
If you just need to avoid shipping the internally named assembly, you may be able to use ILMerge to 'hide' the internal assembly during a post-build step. This is viable if it's just a perception issue for the final assembly names in the binary output directory.
Deal with the issue at the political level by describing your internal library as being no different from any other third-party dependency. Then the naming is no longer a problem. This may solve other problems if you're shipping the source code of this library to multiple clients, as it clarifies that you are not giving full ownership of your 'shared' code to each client. Otherwise they could potentially argue that you are not allowed to use that 'shared' code in projects for other clients, since it is clearly owned by them, having their enterprise name in the namespace.