tune to the new channel that you would like to store. Press and Hold the preset number you want for 2-3 seconds and the preset will update to the new channel.
The SiriusXM home antenna is the best choice, however if you are able to attach a metal plate (4-5" round) to the bottom of the vehicle antenna and place it in a southern facing window it should work as well as the SiriusXM home antenna
The Onyx radio has the SiriusXM tuner built in- there are no "tuner cartridges" to deal with. You can transfer your old XM subscription to the new Onyx plus. Depending on your subscription package, your Onyx plus could also receive more channels.
Yes- you can connect your Onyx Plus with home kit to the radio. The Onyx comes with a 3.5mm to RCA connector, so you might need a 3.5mm to 3.5mm cable if you don't have one.