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 -- Student: I'm new at this university, and I'm lost. Can you tell me where I am? Professor, after a 1- or 2-minute pause: You're in my office. Student: Oh, I see. I'm in the math department. Professor: Yes! How did you guess? Student: That's easy: 1. You thought your answer over thorougly. 2. The facts you stated are exactly true. 3. They don't help me at all!