Changing the fifth postulate does not make it a lie, because lie = false. The fifth postulate is an axiom, which are taken as self-evidently true in that system (or structure). They are, by definition, true and not false (lie). In the case of the fifth postulate in non-Euclidean geometry being "false," yes, the "lie" of the fifth postulate falls apart in non-Euclidean geometry, but that doesn't mean that true and false are themselves subjective as a result. It simply means that you cannot reach true conclusions in both systems in the same way. You're making a logical leap.