The code was already implemented before, which is why these screenshots exist. It really depends of how was the original code kept. If it was overwritten with the lower quality graphics - too bad. If they were smart enough to code the lower quality graphics as a pre-compile option - it should be as easy as pressing a button. If they kept it in any other form, it should be possible without that much work and time, at most one week, and it'll be worth it.
Edit:
I've come to the conclusion these images are not screenshots. This is how they were made:
1. Oddworld took the 3D source of the CAM files and rendered it at 640x480 (With slightly different lighting)
2. They did the same for the sprites.
3. Someone quite unprofessionally combined these all together.
These resulted in a few problems:
- The muds are much taller than usual
- Some objects (Like the wheel in the attached example) are mis-positioned and mis-scaled.
- Some objects (Like the meat grinder blade in some other example) are missing.
- The font for the LCD is different
- The laughing gas looks very different from the game.
The bottom line - there's no high quality version of AO and AE, and there will never be.
This does not apply for all beta images though! Only images with high quality graphics are "fake".