MusicBrainz Web Service outputs release packaging MBID under 'packaging-id' key in JSON format. MB Solr search server should use the same format to preserve backward-compatibility.
In JSON terms, packaging returned in release search results should change from:
It indeed is an active API break, but this piece is not known to be widely used at the moment. Since MB API clients are more often correctly handling lookup/browse results, they are most likely already able to handle both packaging formats in release search results (MusicBrainz Server has been updated to handle both already), or at worse the potential rare API clients querying release search only have been given a month to handle them.