substitute server rename

[Andy Tai]

Hi, I have another guix channel in my system configruation which has
its own substitute server.  Recently the substitute server have been
renamed so following

https://guix.gnu.org/manual/en/html_node/Getting-Substitutes-from-Other-Servers.html

I edited /etc/config.scm to rename the substitute server.  Then I do

sudo guix system reconfigure /etc/config.scm

but that failed because it still tried to reference the old substitute
server which no longer exists with its host name unresolvable via DNS
and thus the reconfigure command then failed.    What can I do to get
around the old substitute server no longer available?