It will really mess up a document that is not formatted using Styles, especially one where you press Enter at the end of every line. The reality is that these fonts take up different space on a page. Courier New is a monospaced font; TNR is proportionally spaced.

You may have some work ahead of you. I would recommend that you read this short series of pages before you start. It should take about an hour and will save you weeks of work. Basic Concepts of Microsoft Word - from Shauna Kelly