As I said on the mailing list, I don't think interfaces should be turned to abstract classes. This is a wrong design decision from a pure programming view point as interfaces should expose what abstract classes implements. They are complements and not competitors.
I think we should have a SolrServer interface and an AbstractSolrServer that implements it.
Methods should be added to AbstractSolrServer on minor releases, and on each major release, the interface signature should be changed to expose them in an adequate manner.