Here is a link to show a little of the installation process.
I am not familiar with this product, and not necessarily recommending it, but it gives a basic overview of the process, and the difference between full replacement and inserts.
WARNING, HAS OBNOXIOUS MUSIC AND RAH RAH SALESMANSHP.
A lot of times you will pay less for an independent contractor than having a major company redo them- Renewal by Andersen, Pella, Marvin. On the other hand, they are large corporations that try to maintain a high level of customer service. Make sure whoever you hire has done this before, is licensed, has extensive references, (check them) and has been in business for at least 5 years.