About a week ago Australian TV News Channel 24 had an interview with the Black box CEO. In that interview the CEO stated that the Black box in MH370 was a 2006 model which had a 6 year replacement cycle. He stated to his records they had not replaced the batteries or serviced that equipment. Maybe the battery life as stated by Malaysia was the absolute last usable date not the manufacture's recommended maintenance schedule.
Just because the manufacturer has not seen the ULB, does not mean the battery has not been changed. Dukane will change batteries, but the also sell spares kits, and a suitably qualified component workshop (which most global airlines will have at their disposal) is more than capable of replacing the battery using those spares kits.
The MAS CEO stated that according to the maintenance records, the battery was due for replacement in June 2014. That record will have been closely scrutinised already by the Malaysian CAA. At this time I have no reason to doubt it.