In a recent interview by the Japanese Impress photo magazine (Issue No. 1 of 2014) with the Chief Sales and Marketing Manager of the Japanese Ricoh Imaging, it is hinted that the 3.5 years old 645D will possibly be replaced in 2014 in due course:
http://www.impressjapan.jp/pdf/271401/1113110212free.pdf
(in Japanese, Warning: Very Large File in 30MB!)
It is asked by the editor that if something more concrete would be disclosed as early as CP+, the Ricoh official refused to respond and didn't tell anything (but just laughed). Btw, it is guessed by the magazine editorial that the 645D replacement should have a change in sensor format and they even predict that it could use a Full Size 645 sensor! Oh really?! :-o