You can change the contact source for an automated message group without reconfiguring the message.
The criteria of a query that is associated with a message group can be changed at any time. However, the contact source that is associated cannot be changed from one query or database to another.
The same applies to the contact source associated to a message group email, which can be automated against a query. The criteria of this query can be changed, so the next time the email sends it will reference the current criteria of the query contact source.
Acoustic Campaign does not retain all past criteria after it is changed, so if the query associated to a message group email is modified, Support cannot troubleshoot why certain contacts did or did not meet the criteria of the email that was sent before the last configuration date of the associated query.