lightning network – Is it possible to change `their_to_self_delay` after a channel is open?

Rene said here that a c-lightning node is safe for around 24 hours to be offline (depending on the locktime). This is set with watchtime-blocks in the configuration file of c-ligthning. But this will only be used for new channels, as far as I understand.

Is it possible to change the their_to_self_delay value of a already existing channel?

The their_to_self_delay value is equal to watchtime-blocks value set in the general c-lightning config file.