Hey there Steve, glad to hear the fish are killing your new mix!
I would just add the new canister to the existing set-up (continuing whatever filtration you currently have) for about 3 or 4 weeks and then run the canister alone.
If you drop off your other filter and add the new one, there will be some trace off ammonia building up and nitrite will eventually build as well. Small amounts of ammonia have been linked to improper growth in juvenile fish. The nitrite can kill at a small amount so you would have to track really well and use salt. I would prefer to skip any of those risks and just use the 2 filters for a month. Now that you are using a beefheart/seafood mix thats a bit messy, that will only add to the ammonia/nitrite build up.
Hope that helps,
Eddie