A change in style in a comic over a long period of time is not unusual. If you read Ah My Goddess, Maison Ikkoku and other long series, compare the style in the first volume with the style in the later volumes.
Bear in mind that Erfworld has been drawn in the same sort of time frame - we have had a single comic appear in the same period of time that it took several volumes of AMG to appear.
But, as has already been said, you can equally attribute the shift in style to Parson's changing perception of Erfworld. The arguement against that is that we saw Erfworld long before Parson did.