Seeing apt-get changelogs for to-be-upgraded packages

The nearest thing to what you are looking for can be had with a package called apt-listchanges, which will give you a changelog summary when you upgrade packages. You can set it up so the changelog notes appear in the terminal directly after apt-get downloads the upgrade (see below explanation and screenshot), but before you install it.

You can install the program with

sudo apt-get install apt-listchanges

and then set it up with

sudo dpkg-reconfigure apt-listchanges

The setup file created is /etc/apt/listchanges.conf.

My setup has the changelog information being shown directly as text in the terminal (stdout), which is just what you wanted. I find this is better than having a pager load up the information. I just have changelogs selected in my conf file, so the news about the package is not shown. I have also set it up to email root the excerpt of the changelog. You need to set up local email by referencing my article here if you want to use this feature.

This is my /etc/apt/listchanges.conf:

[apt]
frontend=text
email_address=root
confirm=1
save_seen=/var/lib/apt/listchanges.db
which=changelogs

A screenshot of the upgrade procedure when apt-listchanges is installed. I have set it to ask me for install confirmation after reading the changelog excerpt.

For more information see man apt-listchanges and the Ubuntu manpages online.

I use aptitude:

$ aptitude changelog package-name

See this question: apt changelog for to-be installed packages

$ apt changelog firefox

Works for me. For external PPAs it fails, even though this one is at http://changelogs.ubuntu.com/changelogs/binary/p/plasma-framework/5.67.0-0ubuntu2/changelog

$ apt changelog plasma-framework
E: Ophalen van changelog:/plasma-framework.changelog is mislukt  Logbestand met veranderingen niet beschikbaar voor plasma-framework=5.67.0-0ubuntu2~ubuntu19.10~ppa1

Or more useful, via Synaptic:

This change is not coming from a source that supports changelogs.

Failed to fetch the changelog for plasma-framework
URI was: http://ppa.launchpad.net/pool/main/p/plasma-framework/plasma-framework_5.67.0-0ubuntu2~ubuntu19.10~ppa1_amd64.changelog

Which doesn't work:

E: ERROR: couldn't open /root/.synaptic/synaptic.conf for writing - ofstream (5: Input/output error)
E: An error occurred while saving configurations.