[jrv]

First approaches are happening for this now:

https://groups.google.com/forum/#!topic/prometheus-users/wO5...

- https://github.com/Percona-Lab/prom-migrate (requires old Prom server to run for reading out data)

- https://github.com/juliusv/prom-data-migrator (operates offline on old and new storage dirs directly)