Lots of people I know use a 4/3 as a transitional suit. It's also known as the "luxury suit." I've always gone from my 3/2 to my winter 5/4.
One other option I heard of is to get this thing, thicker than a rash guard and with a hood, that is just for the upper body and goes under a 3/2. In this way, you can spend $80 instead of shelling out the $400 or so for a new suit.
well, Im not a wetsuit pro, but Im in ny and I use a 4/3 billabong sg5 and was in this past week. I super warm the whole time and could have easily spent as long as I wanted with out being cold.
billabong charts have the suit for water temps as low as 50degrees...