Complex sentence: They occupied the lowest officer posts until a normal officer rank was found for the position, and those who finished the officer preparation with a minimal passing grade and were substituting in command positions, were eligible for promotion to normal officer rank after a set period.
Split into simpler sentences: They occupied the lowest officer posts until a normal officer rank was found for the position. Those who finished the officer preparation with a minimal passing grade and were substituting in a command position were eligible for promotion to normal officer rank after a set period.