You can use several accounts on the same device, but each one will have its own subscription and recommendation settings. When you change accounts, all tracks that you downloaded for the previous account will get erased from your device's memory.
If you registered several account, you can switch between them but they will not sync.
If you signed up for a subscription on an account that you don't use, email customer service and we will transfer the remaining paid days to a different account.
It is not possible to transfer tracks that you downloaded You can, however, copy your playlists to a different account.