The web machines are not being migrated. This announcement only refers to the mail machines -- so there's little chance of any problems with your scripts (perhaps Sage should have been more specific).
We have about 4 web machines that run Postfix; all of the other web machines run generic Debian Sendmail.
There are actually one or two snafus related to switching to Postfix (these would only be a problem if / when we switch the customer web machines over) -- the only one I'm aware of is that the Postfix "sendmail" command will fail if there's an address specified after '-t'; the real sendmail won't fail, but will do different things depending on version....
aura% /usr/sbin/sendmail -t email@example.com
sendmail: fatal: cannot handle command-line recipients with -t
aura% /usr/libexec/sendmail/sendmail -t firstname.lastname@example.org
[ type in headers ].
-t means "specify the headers in the message" - so if postfix were to allow you to specify a recipient address in the command line, it would have to figure out what to do if a recipient was specified here and in the headers (Sendmail does different things in this situation depending on the version).
When we migrated that small cluster over to Postfix, we had one person writing in to complain about this problem, however I don't think it's that common.
We also had a couple complaints because /usr/lib/sendmail (as a link to /usr/sbin/sendmail) got removed. While /usr/sbin/sendmail is the path we tell people to use, /usr/lib/sendmail should work as well. If we were to migrate the web machines to Postfix in the future, we would make sure to add this link.
For the record, I'm very excited about this migration - we've already moved most of our customers over to the new system, and aside from various migration-related confusion, things seem to be running much more smoothly.