Switching between numeric and authoryear in Natbib

Particularly I find this somehow awkward since it will disrupt the consistency of the citations. That being said, one option would be to use the numbers package option for natbib and to use \cite the first time that a bibliographical reference will be cited and then a combination of \citeauthor and \citeyear for the subsequent citations of the same bibliographical item; something along these lines:

\documentclass{article}
\usepackage[numbers]{natbib}

\newcommand\Mycite[1]{%
  \citeauthor{#1}~[\citeyear{#1}]}

\begin{document}

First citation: \cite{goossens93}
Second citation: \Mycite{goossens93}

\bibliographystyle{plainnat}
\bibliography{biblio}

\end{document}

with biblio.bib:

@book{goossens93,
    author = "Michel Goossens and Frank Mittlebach and Alexander Samarin",
    title = "The Latex Companion A",
    year = "1993",
    publisher = "Addison-Wesley",
    address = "Reading, Massachusetts"

}

resulting in:

You may use [\citenum{author99}] for number citation, and \citet{author99} for author-year style.

Look at https://gking.harvard.edu/files/natnotes2.pdf