How to temporarily switch back to previous behaviour of latex?
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samcarter 
*Background story: To find out when some problem/bug first occurred, I often do tests with previous versions of a package/class. For example this week I wanted to test something with beamer v3.51, but unfortunately this version is no longer compatible with current latex due to changes in the hook management.*

Question: how can I temporarily revert such changes without switching back to an old texlive, because this will change versions of all other packages as well, making it harder to debug the problem?
Top Answer
samcarter 
By chance, an email by Hironobu Yamashita just had the solution I was looking for. With `\RequirePackage[2020/02/02]{latexrelease}` one can temporarily revert the behaviour to a previous state.

MWE to compile a locally checked out version of beamer v3.51:

```
\RequirePackage[2020/02/02]{latexrelease}
\documentclass{beamer}

\begin{document}
	
\begin{frame}
	abc
\end{frame}	
	
\end{document}
```

Caveat: this will of course only work if none of the used classes/package relies on the new additions
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