I'd recommend taking a snapshot/copy of the VM and trying to upgrade it directly from Debian 6 to Debian 9. If there's only a few minor problems to fix up and it still works, go with that. Otherwise revert to the original, and upgrade it one release at a time - you'll have more to fix up at each stage but that's the officially recommended and supported procedure (upgrading from one debian release to the next is pretty much guaranteed to work. upgrading from release n to >= n+2 is not).
Always test major upgrades on a test machine/VM before risking your production server
BTW, postfix is pretty good about backwards compatibility. Not sure if it's a debian default or an upstream default, but it runs in backwards-compatible mode until you tell it not to. e.g. from a machine I haven't bothered checking/updating the postfix conf for:
$ postfix reload
postfix: Postfix is running with backwards-compatible default settings
postfix: See http://www.postfix.org/COMPATIBILITY_README.html for details
postfix: To disable backwards compatibility use "postconf compatibility_level=2" and "postfix reload"