It's my understanding that the accounts would be just fine and the passwords from AD would override any passwords that were there. It could change their name and profile info if it's different in the AD sync. But all of their data and stuff is untouched, it'd just say hey you already exist, nothing to do here.
Really, I think AD Sync is simply a comparison of the two and then a list of changes whcih then gets uploaded. So (I think) the AD sync really just creates a bulk upload file and uploads it...of course it's done automatically.
I'd wait until others reply before trying it :)