Halliday (2008: 86-7):
But given that “system” is simply the name given to the potential that is instantiated in text, when we talk of the system changing what we mean is that the text potential has undergone a shift of some kind or other. This may be quantitative change, a shift in the probabilities of the terms in some grammatical system … ; or it may be qualitative change, as happens when a term disappears altogether … . Quantitative changes may stabilise; or they may move on to become qualitative … .