Dear Stewart, even w/o discounting the mess that Heuer was in at the time, it is not so uncommon for a brand to change the crown design from screw-in to push-down.
As far as I know, there was another diver watch from Heuer that shared the same fate though I cannot remember which one.
If I think of the various watches I've been infatuated with, I can remember at least two that had the switch over time:
- the Fortis Cosmonauts Chronograph: early L5100-based have the screw-in crown, later ones and V7750-powered ones have not.
- the Sinn 142: the other way round, first issues have the push down and later ones the screw-in version.

Typically though the change is in the direction you're pointing, ie.from screw-in to push down: that's because over time a screw-in crown does tend to wear out and further more modern crowns are able to withstand pressure better even without such feature.

Cheers,
Fabrizio