Device Already Associated Message

i just upgraded from an iPhone 4s to a used iPhone 6s. I did a back up from iCloud and had no issues transfering everything. I clicked on a song in my iTunes to download it and got the "This device is already associated with an Apple ID" message. Just trying to listen to a song through the cloud did the same thing. It gives an option to transfer and something about if you download past purchases with your Apple ID, you cannot download past purchases with a different Apple ID for 90 days. What would happen if I hit transfer? Does this just move the song to my new device and then I wouldn't be able to move it elsewhere for 90 days, or something else entirely? Also, this was someone else's iPhone, but as far as I can tell everything has been wiped and I'm logged into my iTunes and iCloud accounts. I've only had one Apple ID and its been for nearly 10 years, so it's not a transfer between my Apple IDs issue.