You don't switch.
To get a long cat burn, you simply start the fire, get the flue to 500 internal temp and engage the cat, then slowly close the air down until you have minimal or no flame. You should then have a low cat burn for the duration of the burn. Because the stove is designed to always burn in the most efficient mode, you may get moments during the burn when it will switch to secondary burn for a bit, or when both secondary and cat burn may be taking place concurrently. But you will have a long, catalytic burn.
It is easy to control the type of burn you get in the PH. You use split size, quantity of wood, and amount of air
to regulate your burn. It is easy to learn which combinations give you, in your climate and with your chimney, what burn in a given weather period. Once a fire is established and set to your liking, it will continue to burn very evenly for many hours.
A nice feature of this stove is its amazing responsiveness. If you have a low cat fire going, then decide you want to sit around the fire and enjoy a beautiful fire show, you can simply open the air a small amount. You'll immediately get an amazing secondary fire. When you want to go back to a cat fire, you just shut the air back down, and she settles in for the remainder of the long slow burn. Very nice stove.
There are no tubes in this stove. There is a stainless steel secondary burn plate.