What was most likely done was to input the front LCR channels into the 5.1 exactly as they were, without alteration, and drop the Lc and Rc due to not being needed. The mono surround would go equally into both rear channels at -3 dB, so that when played back out of both speakers it would maintain its proper level. It is possible that some sort of de-tuning or other de-correlation effect may have been applied to the rear channels to prevent them from forming a phantom center image in the back of the room, thus making the surround effects more diffuse and less localized. (THX home video releases of old sound mixes have done this, but I don’t know if any other companies ever did it.) The original didn’t have an LFE channel, so it may have been left blank.

I haven’t actually heard the original mix of 2001, so this isn’t a firsthand account of how it sounds; it’s just what I’ve learned about how these formats tended to be dealt with. I do know the 5.1 on the Bluray sounds rather good, and its aural aesthetic matches my memory of watching the film on VHS—I think its changes come mainly in the form of the directional dialog having been all moved to the center, and the addition of some bass to the ending sequence. I’d certainly like to hear the original and compare them directly at some point.