No guarantee of big improvement. Coached a kid who ran 1.47 in HS (same year as Brazier). was doing very little training (as a 400/800 guy) with NO training partners, went to a reasonable college program, got up to 70miles/week, but never broke 1.46. Is now a 1500m runner. Flatt is much faster than he was, but so was Abdallahi Hassan (Wisconsin) - he ran 1.47 a few years ago in HS off of a 46.8 400m, but has yet to break 1.46.
Brazier running 1.43 as a Frosh (record at the time, maybe should be still given Saruni's situation) was huge. But he has taken just over a second off of that since, and is having trouble staying healthy. Murphy hasn't improved his time since that OG 800m in 2016. So progression isn't always linear (as stated above), and it is easy to stagnate in the 800m.