Most scholars (such as Ibn al-Jawzi) are against using the names Malaak or Malaika, which mean "angel". The reason is that these are feminine-sounding name and when people use them for girls, it strengthens the image that the angles are female, which is against the Quran, as shown in the following verses:
[panel_pink]Verily those who believe not in the Hereafter name the angels with the names of females. They have no knowledge to base this on. They merely indulge in guess-work which can never replace the truth. So ignore those who turn away from Our revelation and seek nothing but the life of this world. (Quran 53:27-29)[/panel_pink]
This is not a big issue and if someone is already named Malaika, there is no need to change it. But it is best to avoid this name because of its pagan connotation. When you name a girl "angel", even though angels are not female, it is can be considered an act of disrespect against the angels and the Quran.
There are many scholars who themselves have names that are not recommended by the scholars, but they didn't change their name because once a person has a name, the name refers to the person, not to the meaning of the word. The rules regarding which names are good only apply to naming new babies. Once a person has the name, there is no need to change it.
The only names that have to be changed are those that contain a clearly forbidden meaning, such as Abd al-Lat ("servant of al-Lat", al-Lat is the name of a pagan goddess), these names are called muharram ("forbidden"). As for Malaika, it is called a makrooh name ("not liked by scholars"), which is a different category and changing it is not necessary.