> arthur told me that can arise rounding errors.

 You have to remember I used to be a mathematician.  I say things that
are meant to be true, not meant to be helpful :-)

 If you want to be sure that LuaTeX's output looks right in the most
recent versions of Adobe Reader, editing the font is the way to go
(although there might be license issues with commercial fonts).  Of
course conversion is not perfect, but you can't have it both way!
Besides, the metrics won't change if you only modify upm; only the
outlines do.

> I should try to convert to Type1 instead

 That should indeed work if you don't need more than 256 glyphs in the
font (are you *sure* you don't?).

       Arthur