But yes, when the database was converted to its current format some of the old data was not imported, and thus old games do not have units.
Also, a MAJOR MAJOR change was the introduction of draws. Not sure when they came in (and at first it was only if you asked Kestas to draw your game for you, then forum threads, then /draw in chat, finally votes) but before that games would go on for ages in stalemate until someone gave up / went away (eg
gameID=333). Similarly, change of phase length reduced the number of bad results.