I think you shouldn't worry too much.
This is probably just the kinda stuff you already read, but: placenta previa is a condition where the placenta is low lying in the uterus - it can either cover the cervix or just be very close to it. The cervix gets very large in pregnancy, esp in the early stages when the baby is small, it makes up a large portion of the total size of the uterus - its the slender part of the pear at the bottom. Usually as the uterus grows, the placenta will move up the side of the uterus. The placenta will increase in size a whole bunch, so it makes sense that it can move where it needs to. It doesn't need to detach for this purpose as it's building and rebuilding itself while it's growing. A placenta previa that is merely close to the cervix but not covering it can result in a natural birth, also.
I think it's a little alarmist of your mw to tell you this, but not to tell you when they have information, even if it doesn't change your condition or care is a morality issue. If it does change your care, it's a serious issue. However, telling you about somehting that may not be a problem can cause problems just by having you worry about it and set your mind on the outcome.
So, I suggest that you acknowledge this may be the case, but don't dwell on it in any negative sense. You're right, this is early in your pregnancy for this to make a difference in the final outcome. Also, you may want to spend some time visualizing your little bud growing, along with the placenta providing nourishment to him/her, and the placenta growing healthy and strong where it needs to (not tell it where it should go, but encourage it to do the healthy placenta thing - of you get my drift).
You're not the only one who's asked this question, so it does come up. And I can't recall anyone saying it continued to be a problem.