The current version of FileMaker Pro is 9.0v3, and it will convert FileMaker Pro 6.0 files to the current version. When you first open a 6.0 file with FileMaker Pro 9, a message appears that the file will be converted. You will have your old file (in case you want to go back to 6.0), but a new file will be created with the extension .fp7 (Version 7, 8 and 9 all have the same file extension).
If you need additional clarification, please let me know.
or drag any related fields over the Filemaker Pro icon and it will mostly preserve your file relationships and wont rename your old files.