If you already have both cards then you should just keep the 7870XT and be done with it. Like you said the 1gb memory worries you and your already dealing with it's limitations....Things will only get worse in the long run.So I've been experimenting with the 7850 in all my games and not really noticing many slowdowns (monitored via FPS). I've swapped the 7870XT in there a few times, but haven't seen too much of a difference.
I have noticed that in Civ 5 I'm getting the "gray tiles," indicating that there isn't enough VRAM to hold all the textures at once. It makes me a little nervous about the longevity of a 1GB card. But the cheapest 2GB card is another $15, plus I would have to deal with another RMA.
So at this point, I think the 7850 is fast enough for me, but the 1GB still worries me a little.
If both games came with the codes then just cash them out. Why deal with a rma at all? You could most likely make your money back on the 7850 by selling the card and the (2)codes anyways....Maybe turn a little profit once you get the rebate back.
Later on you'll be doing the " I shoulda " , " I coulda had " thing if you don't most likely.