As 'Acceptable' is a passing grade, the student might be able to enter into the N.E.W.T. class that they passed in. However, some teachers, like Severus Snape, only allowed students with an 'Outstanding' grade to enter his N.E.W.T. classes. Additionally, Minerva McGonagall refused to accept Neville Longbottom into her N.E.W.T.-level Transfiguration class with only an 'Acceptable' on his Ordinary Wizarding Level. This may indicate that students who receive only an 'Acceptable' O.W.L. for a subject might not routinely be allowed to continue at N.E.W.T.-level, as Professor McGonagall believed that Neville would not be able to cope with the course load. It is therefore also likely that, given that a student has already passed a subject with an 'Acceptable' grade, that the student could not retake that subject.