Can I replace "change into" with just "change"? I don't see why the word "into" is necessary in this sentence.
No, it would not be the same to replace "change into" with just "change". If you said "change your swimming clothes" here, this would imply that you already have on swimming clothes and you are just going to change them and put on a different set of swimming clothes. "Change into" describes the action of changing into the swimming clothes, assuming they aren't already wearing them. Ex: Hurry and change into your school uniform, we're going to be late for school. Ex: She changed into her pajamas and went to sleep.
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