The 2024 U.S. presidential election has sparked widespread online discussions about the presidential candidates. Joe Biden's withdrawal from the race and Kamala Harris's subsequent entry as the Democratic candidate likely alter the dynamics of these online discussions; yet, this hypothesis requires evidence. Here, we study how sentiments and user engagement in social media posts mentioning presidential candidates change after Biden's withdrawal. Our analysis is based on N=680,609 TikTok videos that have accumulated over 4 billion views, with more than 23 million comments, 31 million shares, and 335 million likes from November 1, 2023, to October 6, 2024. We find that: (i) Before Biden's withdrawal, video posts mentioning the Republican candidate (Donald Trump) have higher positive sentiment and lower negative sentiment compared to those mentioning the Democratic candidate (Joe Biden). (ii) Following Biden's withdrawal, positive sentiment in video posts mentioning the Democratic candidate (Kamala Harris) increases by 46.8%, while negative sentiment decreases by 52.0%. (iii) Regarding user engagement, before Biden's withdrawal, video posts mentioning the Democratic candidate have 64.9% higher odds of being shared and 39.5% higher odds of receiving likes compared to posts mentioning the Republican candidate, with similar odds of receiving comments. (iv) After Biden's withdrawal, the odds of being shared increase by 53.3%, and the odds of receiving likes increase by 77.4% in both video posts mentioning the Democratic candidate and video posts mentioning the Republican candidate. Our findings offer insights into how sentiments and user engagement in online posts about the 2024 U.S. presidential candidates shift following Biden's dropping out from the presidential race.