For some reason my initial reaction to this is always to translate it as "she has to get dressed" instead of changed. What is the verb for getting dressed (or dressing oneself, since I'm guessing it's reflexive)
- He has to get dressed - Hij moet zich aankleden
- He has to change - Hij moet zich omkleden
Why not "She must change the clothes"? If it is not written, whose clothes are these, then "the" means that these are her clothes, so the meaning is the same. There were many such examples in different DL excercises, that "the" even had to be added, so that one knows that these are specific clothes, of specific person.
"omkleden" always refers to one's own clothes, unless a different object is mentioned in the sentence:
- Ik kleed me om - I change (my clothes)
- Zij gaat zich omkleden - She is going to change (her clothes)
- Wij moeten ons omkleden - We have to change (our clothes)
- Zij kleedt de pop om - She changes the clothes of the doll
As you can see, if there isn't a different object in the sentence, a reflexive pronoun is necessary.