If they are closer to beginner 400 runners, I’d leave the SE at 300m and increase the RI. If I see a huge drop in times, I’d perhaps move to one rep a bit sooner in order not to fry the nervous system before getting the results I really want in the 400s. Again, if times are dropping there rapidly and there are meets each weekend, I’d consider reducing the SE to 1 x 200m to keep fresh for the weekend meets and I’d definitely look to reduce the number of 400s, particularly before an important comp, moving to 200s, and then go back up.