A channel can be updated as long as the number of subscribers to that channel is equal to one, which means that the channel owner is the only subscriber of the channel.
The channel becomes immutable as soon as a new subscriber joins the channel, to avoid, constant changes on the channel (even with a single subscriber). A fee of 10 DAI is charged that goes to the Fee Pool.
The penalty fee can be adjusted in the future before going to the mainnet. After mainnet, it can only be adjusted by the voting mechanism of the protocol.