I’m also nervous about breaking migration through the archives.
Unfortunately I don't know enough about HyperKitty to help you there. I suspect that the HyperKitty guys have paid about the same amount of attention to consistency across migration as the Postorius folks have, and it matters more there because who knows what people might have bookmarked?
We have a way to redirect the old archives URL format to the new one, but it's not very solid because it depends on the order the emails were archived (that's how it was done in pipermail), and a duplicate email can put the numbers off. If anyone has a better idea, I'm interested, I'd sure like to preserve all the links to individual emails that are laying around on the interwebs.
Aurélien
http://aurelien.bompard.org ~~~~~~ xmpp:aurelien@bompard.org Better to light a candle than to curse the darkness. -- Chinese proverb