They have said your guardian will be able to transfer from last gen consoles to the new gen only by brand though so PS3 to the 4 and 360 to the One. However once the transfer is done it can't go back ao you cant go back and forth either if you have buddies who have last gen but you wanna play on new gen.
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I think its a great feature myself. It keeps from people not wanting to upgrade to a ps4 because the dont want to leave their character behind.
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This is also something that is common now and pretty nice feature. Assume separate servers then.