Precisely for the latter reason they have a rule that you need to prove that you were in the foreign country for a certain period (6 months IIRC) after receiving the foreign license. I witnessed a very irate British guy at the DMV in Samezu who had gotten his British license two passports ago and had been living in Hong Kong for the past 10+ years; they wouldn't accept his conversion request because he had no documented proof of having lived in the UK after getting the license.