It's perfectly ok to launch some initial lectures and then based on the interest shown by the audience, add new lectures and/or update the existing ones. In fact, many a times instructors update the course - this happens frequently with technical courses. It also happens with some software related lectures series where you may have to update your course just because new version of the tool was introduced in the market !
However, there is a trade-off here! - Those who purchase your course might not be happy seeing what you have launched is not a complete course! This may result in low ratings and not-so-good reviews - that's a possibility!
So yes, you can add lectures, assignments later - your call!