The status of rule reordering

Within the standard theory of generative phonology, the phonological component consists of a set of partially ordered rules; where G1 and G2 are synchronic grammars of a given language L at chronologically earlier and chronologically later stages of L respectively, G1 and G2 may differ in at least the following ways: (a) G2 may contain new (phonological) rules ordered later than all rules in G1; (b) G2 may contain new rules ordered before some rules already present in G1 (c) G2 may contain the same rules as G1, but arranged in some different order; (d) G2 may operate upon a different underlying representation from that for G1 (and hence contain, to a greater or lesser degree, quite different rules); (e) some rules present in G1 may no longer be present in G2 (f) some rules o7f G1 may be present only in a modified form in G2.