No! This can be an evolutionary rather than a revolutionary process. Open source is about giving you the choice. So you decide which products are most appropriate to your needs – and your pockets! Remember...

• Open source software can safely co-exist with proprietary products. There are many open source products for MS Windows. Example: ClamWin antivirus deployed by many small to medium sized companies to replace expensive and flaky proprietary products such as Norton & Sophos...
  http://www.clamwin.com/
• Many open source products are also available for several operating systems including Linux, Macintosh and Windows. For example mySQL relational database system... http://dev.mysql.com/
  & PHP, a powerful web-programming language...
  http://www.php.net/ – both used extensively in interactive websites.
• And an excellent selection of top quality open source software for Windows users is availoable for free download from the OpenCD project...
  http://www.theopencd.org/