Upgrading built-in packages with newer versions in ELPA

[R. Diez]

Hi all:

I am trying to get xref-find-definitions to use several back-ends at the same 
time, because I want to be able to jump to any definition listed in a TAGS file 
(etags--xref-backend) and defined in Lisp code (elisp--xref-backend). 
Sometimes, I make a note of function or variable names in some text file, or in 
comments, so it is not as easy as automatically choosing the back-end based on 
the current kind of source file. Including all Emacs Lisp files in the TAGS 
file with ctags-exuberant is not straightforward, because Emacs installs all 
its built-in Lisp sources in a compressed form (.el.gz files). Besides, Emacs 
will probably index its own Lisp files better than any external indexer.

But anyway, during my investigation, I realised that my Emacs 28.1 comes with 
built-in package xref.el version 1.3.0, and ELPA has already version 1.4.1:

https://elpa.gnu.org/packages/xref.html

list-packages knows that, but it does not offer me an upgrade, probably because 
the version I have is 'built-in', and the other would come from the 'gnu' 
(ELPA) repository:

xref  1.4.1 available  gnu  Cross-referencing commands
xref  1.3.0 built-in        Cross-referencing commands

Is there any way to tell Emacs to upgrade all such built-in packages which have 
a newer version in ELPA?

Or at least, is there a way to let me know? Otherwise, I will probably miss 
such package upgrades.

Thanks in advance,
  rdiez