Most likely because her tastes change, and she decides that she does not like a particular scent. Either that, or she likes it being odorless because she believes that this will help her remain hidden, as there is no scent that can be used to identify her. If this is true, then there is the question why Nicholas did not do the same. I mean, it has been statd that Perenelle wanted to fight, and Nicholas wanted to hide, which is strange considering Nicholas has an odor and Perenelle does not. The most likely reaspn for this that I can think of is that Perenelle wanted to protect Nicholas, so that if Nicholas's aura is smelled, but not Perenelle's, anyone tracking them might think Nicholas is alone, and then be suprised and defeated by Perenelle whom they had thought was elsewere. Also, it could be so that when they perform magic together, only mint is smelled, but the aural power is very strong, too strong for one person, making people think that Nicholas alone is much more powerful than he actually is, and making them more cautious, giving the Flamel's more time to escape.
I do not know, just my