The words "up" and "down" reflect an old view of the world where the heavenly bodies orbit the Earth -- more or less -- and the crystal sphere bearing the Moon divides our world from the changeless heavens. Certainly we could redefine "down" to mean "towards the center of the Earth", but then we have problems when traveling near other heavenly bodies, and other problems that I'll address later. It seems to me that we could reasonably replace the words "up" and "down" with "outwards (from the Core)" and "inwards". This makes for an easier change when discussing other centers of gravity. Besides, it seems self-evident that traveling "inwards" will take you outwards if you keep going past the Core of the Earth, whereas language gives us no reason to think that going "down" will ever take you "up". (In this connection, consider the possibility of launching a spacecraft horizontally from a mountain.)
This can help us in other fields as well. In NetHack and other RPGs, we can describe dungeon levels as later vs. earlier and experience levels as greater vs. lesser, instead of using gravity-related words. Thus, we could say, "go forward a level," or "increase a level."
If we could agree on one or two further conventions, we could even classify "mystical" experiences in value-free terms. For example, if we accept the kabbalistic classification system, we could speak of them as Earthwards vs. One-wards (or Zerowards if we accept Aleister Crowley's version.) "Greater" vs. "lesser" would work in arithmetic, but here they have too many other connotations. People speak of the One as "greater" and "higher" than other numbers. I'll explain the kabbalistic part later for people scratching their heads at this point.