Hey all, sorry for not responding :-/ I am now father of a lovely boy, so there is not that much time left for smart home anymore.
@christoph_wempe : I’ll improve the readme with your suggestions.
@greg: I understand your wish, but I think I will not implement an automated migration mechanism. This is complicated, error prone and in the end the user wanted to have it in a different way. Sorry 
@StephN: You can already specify a retention policy in the configs for all items. For a per-item-config, this is a little bit more difficult. Saving the states in another RP would be possible, but when querying the database, there is no information about the item. This means OpenHAB charts and functions to query a value like <item>.deltaSince(AbstractInstant) would not work anymore for measurements not in the global RP. Maybe a solution for you is saving the data in the global RP in the first place, and then later on transfering them to another RP with a continous query?
@bastian, @klaernie: here you go.
Dominik