thats wrong, you still have close and open a new channel but in the one tx instead of two separate
you can, close, send and open in one transaction. you also can close and open to change the size. you can do other things (maybe) coinjoins or batching between the open and closing. But whatever you want to do, slicing save transactions on chain. You can't avoid to use the blockchain if you want to use lightning non custodial, you at least need to open a channel once.
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