Surely Barbie Girl would qualify? If you listen to the lyrics, and take it in the context of Aqua's output in general, it is intended as a 'proper' song (for them, at least). But from the off everyone pretty much had it marked down as the daft Barbie novelty song (testified to in part by being such a massive hit).
Feel free to disagree, but I at least understand what the OP is getting at and I think others are being somewhat pedantic. They didn't mean that the songs have somehow literally metamorphosed from one type of song into another, just that over the years they have now come to be regarded as something of a 'novelty'/joke/not taken seriously etc in popular culture (ie hence the functions/disco point is an apt one).