https://www.uefa.com/uefachampionsleague/news/newsid=2589547.html says clubs in the Round of 16 may add up to 3 players they acquired in January. These players could have played for another club in the 2018-2019 Champions League and/or Europa League. Other changes are that clubs will get a fourth substitution in extra time and that the Final lets clubs have a bench of 12 rather than 7.