I tried to avoid the code duplicating, so it became more compact and
perhaps less readable.  Also I added reporting about the packages to
upgrade: I thought as they are going to be replaced by the packages to
install, it is ok to add “(removed)” there.  So an output should look
like this (assuming "file-5.17" and "guile-2.0.9" are installed and are
being upgraded):

The following packages will be upgraded (removed):
   file-5.17	out	/gnu/store/...
   guile-2.0.9	out	/gnu/store/...

The following packages will be installed:
   file-5.18	out
   guile-2.0.11	out