Piotr - Just reported the same issue last week - I was very surprised this functionality is gone compared to Rhin previous versions. ( See: Imported block to replace the definitions existing in the file? )
Currently I have no good workaround - with files with 100s of blocks (as in our case) this is a major disaster; ReplaceBlock one by one is not an option; even scripting this seems not that straight-forward. I really hope good solution will be offered soon as this is a major drawback in our workflow here.
~j