Here's what I put in a similar thread about 3 down from this one:
Originally Posted by Jack Frost
Yeah, I thought I was being really clever when I picked up a used copy of the Complete Edition from GameStop. I figured I'd play the 2 DLC I hadn't yet, get the achievements and take it back before the 7-day window expired and get my money back. Free cheevos, essentially.
But then it wouldn't read my existing save. Frustrated I went online and found out that not only does the Complete Edition not recognize save games from the regular edition, but there have been cases where the presence of a regular edition save will prevent the Complete Edition from saving properly. So I was forced to remove my old save.
Then I found out that they integrated the DLC cases into the storyline where they were supposed to fall in the story. Unfortunately, the only two DLC I hadn't played yet were "Reefer Madness" and "Nicholson's Electroplating" the latter of which is now the second to last case in the goddamn game.
So I was forced to play it again pretty all the way through to get the last 10 achievements and 200G. It did nothing change my opinion that this is a terrible "game", either.