No, your placenta doesn't move from being posteriorly positioned to being anterior... So, I'm not sure what's happening there?

Posterior placentas are more common though. Did you read this on the report or did he/she verbally tell you? Perhaps he/she made an error in translation?

Placentas of course grow with the baby - but their attatchment is either posterior or anterior - though some are less so either way IYKWIM?

For example anterior placentas are more likely of course to grow closer to the os - simply as they are closer of course being anterior. But as many know a placenta that seems close to the os in a 20 week morphology u/s can & very likely will grow away from the os...

I can't explain why this is the case for you. But I will be really interested to know when you find out!