New PoKeys56U was released recently. They are 100% replacement for PoKeys55. Mechanical dimensions are the same, all software which works on PK55, should work also on PK56U. New PK56U has more procesor speed and it is prepared for upcoming aditions.
Here is also comparision between all PoKeys models: http://www.poscope.com/uploads/files/PoKeysComparison.pdf