fontspec breaks capital greek letters redefinition

I am unable to redefine uppercase math symbols when using the fontspec package.Here is a minimal working example:

\documentclass[ngerman]{standalone}\usepackage{fontspec}\let\psi\alpha        %works\let\Psi\alpha        %does not work\begin{document}$\psi \Psi$\end{document}

using \let just ignores the redefinition.using \renewcommand yields an error saying that \Psi is already defined.

how can I avoid this behavior?

this seems to be related tofontspec breaks capital greek letters in DeclareMathOperator