How to properly upgrade Guix System & Home?

[Tanguy LE CARROUR <tanguy@bioneland.org>]

Hi Guix,

For quite some time, I'd been religiously upgrading my system weekly.
My mantra was something along those lines:

```console
$ sudo guix system delete-generations
$ guix package --delete-generations
$ guix gc
$ guix pull
$ sudo guix system reconfigure my-system.scm
$ guix package --upgrade
```

I was happy with it, but two recent events made me question my faith in this…

1) I realized that there was a third `delete-generations`!

Believe it or not, I didn't know about `guix pull --delete-generations`.
Actually `guix pull --list-generations` was listing a **lot** of month
old stuff.

2) I migrated to Guix Home!

Suddenly everything I thought I understood didn't seem to make any sense
any more! For instance, I can do `guix home reconfigure` and `guix gc` in
a loop and `guix home reconfigure` will keep on downloading things and
`guix gc` will keep on deleting them!? And when I say "things", I mean
packages that are explicitly declared in `my-home.scm` and that I actually use!
When I compare `ls -l (which imv)` before and after a cycle
(`guix home reconfigure` / `guix gc`), I have the same hash.
Which make sense because I haven't `guix pull` in between. So, why is a `imv`
garbage collected and re-downloaded/installed?!

Does it make any sense? Am I the only one facing this issue? What's the
proper way of upgrading a Guix System & Home?

Any help would be very much appreciated!

-- 
Tanguy