Every recast makes the character a bit different. And it happens all the time. Stephanie, Abby, Brady, (both Jack and Jennifer), Billie, Austin, Shawn, Belle, Mickey, Will, Philip, Mike Horton, and yes, even Roman.
With Ciara, the same actress had the role for a lot of years, (unlike for instance, Will, or Allie, who were portrayed by several different actors while growing up)....and yes, she was so good...that was why she stayed so long. But, there is no way to turn 12 yr. old Lauren Boles into being 16 yr. old Ciara. I think MAYBE the casting focused more on resemblance than anything else. But, who knows what factors were taken into consideration.
But you know, many times, actors come on the show, and learn a great deal from the older vets. After a year or so, the improvement shows. I know I have read comments from some actors who were so grateful to the actors portraying either John, or Marlena, or Alice, yes, and Tom as well.