I wrote
\ifx\Umathchar\undefined
\usepackage{lmodern}
\else
\usepackage{unicode-math}
\fi
..
Khaled kindly corrected me off list.
This test file is faulty, prior to 2013 \Umathchar wasn't defined
(just \XeTeX...) so the test file takes the classic tfm font route on
old installations, which accounts for the different output shown on
old releases.
So... this isn't a regression, sorry about the misleading initial message.
But the feature request remains, could it be possible to change the
font parameters after the initial font load so as to adjust the math
typesetting without having to use fontforge or similar to change the
parameters in a copy of the font.
David
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