Migrating sessions from cookie_store to Redis in Rails 3

I need to switch the session store in my Rails 3 app from cookie_store to redis-session-store. There are many reasons for this (security, SSO with other Rails and non-Rails apps). Are there any best practices on how to do it without loosing all current sessions?

What i could imagine is a two steps approach:

1. Collect all user sessions for N days and store them in the DB or in Redis (update if already stored).


2. Use stored user sessions to create entries in Redis.

Alternatively, on the fly migration would also be possible. Means read cookies, use secret key to decrypt the session data and store it as a new session in Redis.