Today I upgraded my main web-host to the Jessie release of Debian GNU/Linux.
I performed the upgraded by changing wheezy to jessie in the sources.list file, then ran:
apt-get update apt-get dist-upgrade
For some reason this didn't upgrade my kernel, which remained the 3.2.x version. That failed to boot, due to some udev/systemd issues (lots of "waiting for job: udev /dev/vda", etc, etc). To fix this I logged into my KVM-host, chrooted into the disk image (which I mounted via the use of kpartx), and installed the 3.16.x kernel, before rebooting into that.
All my websites seemed to be OK, but I made some changes regardless. (This was mostly for "neatness", using Debian packages instead of gems, and installing the attic package rather than keeping the source-install I'd made to /opt/attic.)
The only surprise was the significant upgrade of the Net::DNS perl-module. Nothing that a few minutes work didn't fix.
Now that I've upgraded the SSL-issue I had with redirections is no longer present. So it was a worthwhile thing to do.