/!\ WARNING: I have not yet tested cascaded wheezy -> jessie -> stretch upgrades. As such, I do not currently recommend upgrading jessie machines that previously ran wheezy.

{*} GOOD NEWS: The same steps also appear to work for stretch -> buster upgrades, if proper substitutions are made.

As of 8/25/2018, it works, but isn't seamless. Take the following steps:

If we're doing upgrades like this frequently, I can look into scripting parts of the process.