Use this, in general program mode. Or tweak into another mode, but general program mode is awesome for complex stuff, like combining multiple fields in one using complex regexes and function calls.
It will only parse the first author.
Code:
program:
FN=re(
field('author_sort'),
'([^,]+),.+',
'\1'
);
LN=re(
field('author_sort'),
'[^,]+,(.+)',
'\1'
);
fixed_author_sort=strcat(
uppercase(FN),
LN
)