4D v3.x.x databases can be opened, for conversion, with 4D v6.5. 4D v6.5 will convert the database after confirming that you wish to convert both the structure and data files. The conversion process converts the original files and does not make any backup copies. You will not be able to open the new 4D v6.0.x database structure and data files with 4D v3.x.x once they have been converted. You should make your own backup copies, if required, before converting.
4D v6.0.x databases can be opened directly with 4D v6.5, with no conversion. However, you will not be able to reopen the structure file with 4D v6.0.x. Therefore when you first open a 4D v6.0.x structure file with 4D v6.5, an alert is displayed to warn you of this restriction. Again you should consider making a backup copy of your 4D v6.0.x structure before conversion.
The data file, however, can be used either by 4D v6.0.x or 4D v6.5 without any modification.