The original effects elements probably don't exist at this point.
Yeah, didn't an old friend of ours hear that Trumbull wound up throwing a lot of TMP vfx elements away because of space limitations around the time of BLADE RUNNER? That still puts the blame on Paramount, since they should have wanted all that stuff to store in-house to repurpose.
But that also explains why using stock elements in TWOK looked so bad in the theater, since I'm guessing they were just a copy from the TMP print rather than recomposited.
If he did throw them out, then thats a shame but as I posted above TNG used TMP effects and they appeared to be able to go back to original elements in both 1987 and 1990 and now again in 2012 for the HD Remaster so some if not all the VFX elements still exist somewhere.
I reckon the reason they didn't recomp the TMP shots in TWOK was just to save money and the tech in 1981/82 may not have advanced very far from 78/79 so they may have seen no need to remake it again.