Sorry I didn't get back to this sooner, had some crazy life stuff getting in the way.
I should have said in my original post that there is a reason I'm trying to jump from Wheezy to Stretch. These boxes are in an HA cluster with Pacemaker, Corosync, and DRBD. I have all 3 of these installed from the Debian packages. Pacemaker is not in Jessie (apparently the devs missed the deadline for inclusion), so an upgrade to Jessie uninstalls pacemaker and all of its configuration. That's what I was hoping to avoid, because setting up an configuring Pacemaker always takes me a while to get right, so an upgrade path which does not include the removal of it is preferable.
I did read the documentation and I did try Wheezy -> Jessie - > Stretch first, but the problem was that Pacemaker was removed. Hence, I was hoping to skip Jessie.
If it's simply not possible to do this upgrade then I guess that's that. I'll have to reinstall and reconfigure Pacemaker, which hopefully will continue to be delivered in future releases without fail.