Try selecting a few words that include one of the mark-up styles, then pressing ctrl + spacebar to clear local overrides. If that makes the track changes mark-up disappear, it's as Paul describes. (this also appears to work if someone has been more thorough and used character styles instead of local overrides to mark additions and deletions)

Of course, depending on how well the document was prepared, clearing local overrides may make more changes than you anticipated …