Makes no sense, Birth-sign implies you were BORN under it. For example IRL I was born under the Capricorn, I just can′t suddenly decide that I was born in the summer instead of the winter and have the Cancer or Leo instead. You′re born on a certain day, under a certain sign! Anything else is just time travel!
What they are doing is keeping sign-stones and scrapping birth-signs. I′d like it more if they just scrapped the whole concept of birth-signs and called it power-marks, or something else, but even that would be a form of retroactive continuity since birth-signs have been established quite well and even mentioned by books and characters in game. Heck even Azura herself mentioned birth-signs when she mentioned that the reincarnation of Nerevar was "born under a certain sign".
Yes it′s less flexible to pick them early on but, can′t we then instead have a level cap on the stones and then be stuck with out mark forever after choosing instead ? It could work to just pick one sign and never touch a stone again but before long we will be seeing NPC′s go "Oh I was once The Lover but I went and changed my birth-sign to the steed so now I run much faster", and that, you can not avoid.
You are not changing the sign you were born under -- the birth of your character is completely unrelated to the cosmos in any way, shape, or form. What Bethesda has brought back is the ability to . . . do whatever it is you do at doomstones and get the powers associated with that particular constellation, regardless of the date of your birth.
Multiple times it is mentioned that you are born under a certain star and that your birth-sign is tied to that. You can′t even get past the intro video of Morrowind nor past the tutorial in Oblivion without Azura and Tiber Septim themselves telling you that! Changing a sign you were born under is not possible because you were born under it. Only way to have a different one would be if you were born at a different time.