Question on Dynamic Groups and mappings

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Seeking some help and advice.  We are using dynamic groups and pushing out Learning Plans to our new employees through this functionality.  We've been using dynamic groups for couple of months now.  The dilema I have is that we need to make a change to the Learning Plans that are pushed out to our new employees but don't want those who've already been assigned a learning plan to be re-assigned and pushed the updated Learning Plan.  We only want employees hired after a particular date to have the new version of the Learning Plan.  I'm assuming (and hoping I'm wrong