The closer contact you get with a previously cycled seed sponge the faster it will get the new filter ready. I would take the sponge out of the existing filter and place it below the new sponge in the AC50. That way there is direct contact and flow between the existing bacteria and its new home. If you want you could take the carbon from the new filter and swap it in place of the old sponge from the old filter to take up space, but otherwise carbon is fairly useless. It's still going to take time to get the new filter up to speed on its own but this will help. I'm guessing you're still looking at a week or two before it's ready.
Easing my toes in the water