Closing as the main Solr servers are ready to support different Solr schemas. I.e they have two different cores:
to which CKAN instances can point to.
The CKAN instances that have not been updated (the ones under s004) are pointing to a Solr core with an old version of the schema, so they can wait until upgraded to 1.5.1 to update the solr_url property and rebuild the index.
Data.gov.uk can be dealt with during the next deployment.
It's not clear which Solr server are using the rest of the instances, but they can be updated as necessary when they upgrade their CKAN instance.
