The problem with trying to date the change-over with a serial-number "range" is that during most of 1993, FNH used both the "new" cast-frame and left-over forged-frames simultaneously for 9mm Para Hi-Power production, until they ran out of the older forged-frame at some point in production.
I haven't seen a 1994-dated 9mm Para Hi-Power with an earlier forged-frame, but that doesn't mean they didn't make one. I have seen numerous early cast-framed 9mm Para Hi-Powers made in 1993 that DIDN'T have the serrations on the bottom of the magazine-well. All of these did have the older, original "long" rear-frame-rails and the casting-mark/number inside the frame though. Running change and all............