Mods; please feel free to move this if there is a more appropriate thread.

I have a multi city trip next month HNL-ORD-DTW (6 days), DTW-ORD (2 days), ORD-HNL. There is a strong possibility I may need to drive my daughter's car from Detroit to Chicago. Will it be a problem if I notify UA that I will not be taking the DTW-ORD flight and have them drop that segment? Obviously, I don't want to lose my ORD-HNL return trip. What say you FT sages? Thank you.