The early kits were upgraded in only one way. They had a fairly squared off front strap. They recontoured the front strap. It now fits the hand a little nicer, and they shaved off a little on the circumfrence too.
They still market a frame kit, though they are hard to find. Told and the new are relatively the same, except for the aformentioned re-contour and the grip panels, which are not as wide now.
As for problems, Paras can have some issues that may or may not be a problem for you. Generally, they are a little on the soft side, which means nothing, unless you peen the frame reails down for a fitted slide and frame. They will get loose quicker than a Colt, Springfield, or Kimber. Again, this is only an issue with a tightly fit frame and slide.
Also, whether old or new, they have some parts that are plastic, including the mag release and trigger. Again, a plastic trigger is not really an issue, but the mag release will wear out much quicker than steel. The older kits and some of the newer kits have plastic recoil spring guide rods, which will also wear out much quicker than steel. This does not apply to their assemlbed guns, as they have steel guide rods.
There was a real good article in American Handgunner back in the late 80's on the Para frame. However, I couldn't begin to tell you the year, much less the issue.
Newer frame:
Older frame: