I work for a product design company. We work with (consult for) companies that want to bring a product to market but don't have the internal engineering expertise or capacity to do it themselves. There are a lot of companies like this. They employ some engineers to create specifications and make technical decisions, but not enough engineers to complete the project in a reasonable time.
I have trouble believing that a normal IT department is equipped to do a large-scale migration, especially when they need to continue supporting all their normal business operations.
In summary, if you want to be successful with a large software project--and large software projects aren't what your business does, you probably need outside help that does it routinely. Then the question becomes how you evaluate this outside help's qualifications, how you write your contract, how you set your milestones, and how you communicate your requirements. But that's an entirely different story.