In one of the applications we're working on we recently had to make a move from InfluxDb v0.8.7 to v0.9.6. Because the official migration paths didn't work for us (DB upgrades would either lose data or not finish at all) we had to develop a small c#/.net app that would reliably execute the migration for us.
We successfully migrated around 4GB of data with it and are quite happy with how it went. It did take quite a bit of time but all the data is safe and usable.
The app also lets you specify backfills (rollups) to be created once all the base data is migrated.
Today we're open-sourcing this migration tool in hope it might help someone else make the move as well. :)