Use the "Save as" option that allows you selecting the version of the software you wish to use. For example, a file previously created using Word 97, can be saved as MS Word 97 type of file. The same applies to Excel files created with previous versions of MS Office. Hope this helps.
I just downloaded my new version of MS Office 2010 and my old files converted just fine. If you are worried about that not happening, besure and save your old files on a flash drive or whatever you use to save your files and if they don't convert you still have your old files. Hope this work for you like it did me.