Personally, I'd go with a full rebuild, and mitigate what's possible.
Assuming that sunrise-choir, go-ssb, and similar high-performance implementations will eventually form the backbone for most ssb apps, this should not be a huge problem.
At the moment, #ssb-patchql churns through a gigabyte worth of offset log in a few minutes and stays fully operational for read access in that time. I expect that to become even better over time, and with the "me frontier" offset log from the drank specs thread, it should not even have to be read-only for more than a minute or so.
So this might realistically become a non-issue soon. In the meantime, we can just rebuild.

just my two cents :)