Ah, seems as though I may have found the answer: How to use a struct in an upgradable contract and ZeppelinOS proxy pattern and structs - #6 by spalladino.
Also shedding some more light on this: Upgrades with Peace of Mind: "Structs" Edition, Can you rename variables in a deployed upgradeable contract? - #2 by abcoathup.
I'm getting the sense that the safest means of upgrading would be to add the bytes32 variables after the string ones and leave them intact.