Override source uri of older version of a package?

[RW. S]

Probably not all of this relevant to subject line, but I will give
full context anyway.

I can't git clone anymore because "Problem with the SSL CA Cert". The
weird thing is git (cloning) was working yesterday, and the only
notable thing I think I did was `guix gc` (this is on a fresh
barebones GuixSD install so I haven't really had the chance to do
much).

mark_weaver from irc said GIT_SSL_CAINFO needs to be set, which it is,
to `/etc/ssl/certs/ca-certificates.crt`, but /etc/ssl is a symlink to
/run/current-system/profile/etc/ssl which doesn't exist. Then
mark_weaver said:

<mark_weaver> xd1le: you need to add nss-certs to the 'packages' field
of your OS config to include the CA certificates from Mozilla.
<mark_weaver> and then rerun "guix system reconfigure" and then the
certificates will be there.

So I added nss-certs to packages, then I ran guix system reconfigure.
But guix system reconfigure could not finish because it got stuck on a
download which doesn't exist anymore. Specifically, it got stuck at
downloading nss-3.19.2 via ftp from the following uri:

ftp://ftp.mozilla.org/pub/mozilla.org/security/nss/releases/NSS_3_19_2_RTM/src/nss-3.19.2.tar.gz

And the reason is mozilla has decommissioned their ftp (for this or
whatever): https://bugzil.la/1250774

Now, if you use https:// protocol instead (so just the change the
ftp:// at the start of that uri with https://), it works and I used
guix download to confirm the resulting tarball has the same hash, at
least for this package.

So the question is, is it possible to change the source uri of an
older package? I know I can create a new package object to override
the one that guix has, but I don't want to do that, I want to keep
everything the same, except *if something does need the older
nss-3.19.2*, then it uses the working https:// source instead of the
now not-working ftp:// one. (Note the latest nss is version 3.21, so I
want to keep it like that.)

This is all so that running `guix system reconfigure` again will work,
of course.