Audio survives, which matters more on Reels than you would think
The swap touches video frames only, so the original audio track comes through untouched. For Reels, where a trending audio is often the whole reason the clip works, that means the swapped version still carries the sound you built it around.
It also means lip sync stays correct: the mouth movement is still the original performer's, so the face may be different but the words still line up.