For postfix it will be easy enough, just study http://www.postfix.org/ COMPATIBILITY_README.html.
I went from Ubuntu 14.04 (based on jessie and uses postfix 2.x) to 16.04 (based on stretch, uses postfix 3.x) a while ago, I had a few problems relating to the change from upstart/sysinitv to systemd, but I think Jessie (plain) already uses systemd by default so you won't have that hurdle to jump. Also for amavisd-new I had to change permission of /var/lib/spamassassin directory to 755 - an alternative (untested by me) is to make user amavis a member of debian-spamd group.
