There are a few different philosophies that developers subscribe to when making a sequel. Some make wild design changes, some make iterative improvements, and some leave the basic pieces intact while expanding the narrative. Puzzle Agent 2 is definitely a product of the latter philosophy, and fans of the original will likely appreciate that fact.
For others, Puzzle Agent 2 might be a bit harder sell. Your mileage may vary, and the variance is largely dependent on how tolerant you are of ordering photographs chronologically and sliding block puzzles.