Thanks, Andrew-- I'm still not quite sure what was going on, or why the old project continued to insist on Boost 1.58! *
In the end, rather than migrate the old project, I just made a new project with Tinderbox and migrated (read: copied) over my old code.
This seems to be working perfectly fine, even using my own custom Cinderblock and project templates which haven't been updated in nearly a year. (Though I plan to give those old templates a once-over now anyway.)
*For the record, I was updating from a May 2016 version of Cinder, meaning I was 1524 commits behind master. Probably a good idea to give any legacy projects a fresh foundation with Tinderbox anyway, I'm guessing...