AFAIK, an engine switch isn't as difficult as it seems. All the calculations will be unaffected as it's just a graphical thing (I think ), and textures will remain the same.
- I think the switch will be made after the game is released on Steam.
- No. Most of the core aspects will be unaffected.
- The calculations are all done in LUA format, and they won't be harmed in the switch.
- Nope, models and textures will still work as they did, just with better shaders and lighting.
Basically, what this means for the game is much better graphics, much better performance and much less strain on the CPU and GPU, better shaders, more realistic lighting, and some other artist features that Daffy can work his magic with! And the only drawback is development time I think, so all is well!
Hope this helped!