Oh, and forgot to mention in my other comment.... very nicely done, I have to say. Perhaps this is like when they ascend from princess into queen status? Since they look a bit older and more regal.^_^ Perhaps there's a legend of 'queens of the past', a status that is like a state of being rather than a title, and they've yet to reach that yet?
One way to look at it, is that the rule is sub-species based. Meaning that the ponies who invent new types of magic (alicorns) are the most powerful beings and have the right to rule over the ponies. There is no set supreme ruler, so they all carry the title of "princess" or "prince".
This logic has a few holes in it, but it's an idea.
Oh that logic is a cheese! Holes everywhere.Just look at prince Blueblood, he hardly seem like the inventive type, and then you got Starswirl the bearded, invented hundreds of spells but never became royal. And I heard it said that they are doing this to show little girls that if they work hard enough to do good, they will be rewarded.
Oh, here's a thought for ya. It seems to me that, though less so on Cadence, all three princesses' hair as alicorns are somewhat sparkly and almost partially transparent. As such, how would the mane six look like as adult alicorns?
Also, I've read the synopsis for the next ep, and I theorize the swapping of cutie marks also switches their personalities. Likely to symbolize this, they'll likely change their hair styles with them. As such, imagine how it'd be if it effected all six, not just five like the episode seems to hint at. I could easily see them swapped like this: Fluttershy/Rainbow Dash, Rarity/Applejack, Twilight/Pinkie. XD