Unclear how to set model-defaults from a file

Version: 2.9.29-ubuntu-amd64

I would like to be able to replace by model-defaults using a file as input. The model-defaults help text makes it sound like this is possible:

"""
Model defaults yaml configuration can be piped from stdin from the output of
the command stdout. Some model-defaults configuration are read-only, to prevent
the command exiting on read-only fields, setting "ignore-read-only-fields" will
cause it to skip over the fields when they're encountered.
"""

My reading of that is that I should be able to do something like:

juju model-defaults > model.defaults
$EDITOR model-defaults
juju model-dfeaults < model.defaults

However, this doesn't seem to work in practice. Here's my trying to change my default-series using that method:

$ juju model-defaults > model.defaults
$ grep focal model.defaults
default-series focal -
$ sed -i 's/focal/bionic/' model.defaults
$ grep default-series model.defaults
default-series bionic -
$ juju model-defaults < model.defaults
Attribute Default Controller
agent-metadata-url "" -
agent-stream released -
apt-ftp-proxy "" -
[...]
$ juju model-defaults | grep default-seri
default-series focal -
$

Of course, I know I can change individual model-default values, but I'm looking for a way to load them from a file all at once w/o caring which ones actually have different values.