This is a follow up question to here.
The recommended solution doesn't work for me. I therefore let all my mathematical packages in the MWE. The result is an upright Psi followed by an italic Psi. I would like to have all upper-case Greeks being italic in formulas. The possibility to set one character upright (for a constant, an operator...) would be great. Therefore I wonder, if there is a more beautiful solution around.
\documentclass[]{scrreprt}
\usepackage[final]{microtype}
\usepackage{luatextra}
\usepackage{fontenc}
\defaultfontfeatures{Ligatures={TeX}}
\usepackage[ngerman, english]{babel}
\useshorthands{"}
\addto\extrasenglish{\languageshorthands{ngerman}}
\usepackage{amsmath}
\let\Gamma\varGamma
\let\Delta\varDelta
\let\Theta\varTheta
\let\Lambda\varLambda
\let\Xi\varXi
\let\Pi\varPi
\let\Sigma\varSigma
\let\Upsilon\varUpsilon
\let\Phi\varPhi
\let\Psi\varPsi
\let\Omega\varOmega
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{commath}
\usepackage[
per-mode=symbol-or-fraction,
locale=DE,
sticky-per
]{siunitx}
\usepackage{xfrac}
%
\begin{document}
$\Psi \varPsi$
\end{document}
Edit My knowledge over the used math-packages is quite small. Don't know, if I use them at all (amssymb and mathtools). If they are obsolete or not recommended for typesetting in Germany (English and German) or if something is missing for typical formula setting, I would be glad for every advise.

luatextrainterfers... – karlkoeller Jul 19 '13 at 14:54\usepackage[no-math]{fontspec}, whileluatextraloads it without that option. – egreg Jul 19 '13 at 15:04\let\upPsi\Psi, then do your reassignment for the new default:\let\Psi\varPsi. then you can ask for$\upPsi \Psi$. – barbara beeton Jul 19 '13 at 15:09[no-math]{fontspec}beforeluatextraworks fine. Is that an answer or just a workaround? Should I report the interference anywhere? – LaRiFaRi Jul 19 '13 at 15:12