Depends what is meant by "first pressing." Usually it's the mastering that is changed (notice I didn't say "improved") when there is a later CD
release, but to me, "pressing" is the physical part...and like
@Celt says above, if we're just talking CD masters like the stampers used to make LPs, there should be no difference.
Mastering and newer releases a whole other can of worms, and I've heard both good and bad remasters. Many remasters (especially from the mid 90s onward) got "brickwalled", had the bass and treble boosted way up, and applied horrible sounding digital noise reduction which, once you identify and hear the artifacts, is hard to
unhear. Things in the past few years have seemed to change in the opposite direction and many remaster programs are now more faithful to the original two-track masters than in the last decade or two. There are some out there who are really into "first pressing" CDs and IMHO, some of those are real sonic turds. In many cases, it was a cash grab by the label--grab the first master tape they came across (which was often an LP cutting master, which had all the LP mastering EQ on it), ram it into digital, and slap it on a CD.
One cannot generalize though. Not all early CDs are bad; not all 90s CDs were bad either. It all has to be heard on an individual basis to determine which is the best sounding to our ears.