Luna said if you haven't updated your town for a long time your town might get deleted from the server, thus, changing your dream address when you try to upload it.
This could be an answer, but it doesn't seem likely from the post. Merel was doing it for the daily 5000 bells, so it seems to be that it never gets too long before it's updated. And assuming she had it since the release date of 14th in EU, we've not had the Dream Suite for that long for it to be deleted. Hmm, puzzling!
I also had this happening to me once.. I only play the game on one 3DS and I don't think it's because I didn't update it for a long time..but I remember that it took longer than usual to update my town, to the extent I worried that I might get an error message, and then I was informed that I got a new dream address! Could it be something to do with the wi-fi? I really have no idea..
I haven't had this happen in my English town yet, but the dream address did change for my Japanese town. Since I can't read the language, I never could figure out why that happened and all the possibilities suggested to me at the time did not apply: I updated my town each day (why pass up free bells?), only my mayor character ever took care of updating and I couldn't possibly have used another 3DS because I only have the one Japanese system.
I have indeed been doing this daily from when I got the Dream Suite and just like Lena said I, too, was worrying if I got an error. I don't have a different 3DS system either...weirrrrddd.