It's perfectly possible to transpose music from one key to another without changing it's character; it's no big deal, really, it's all about the tonal range of your target instruments or voices. We are much better at perceiving musical intervals than we are at perceiving specific tones, and transposing a piece preserves the intervals.
Of course he's right; transposing is a trivial musical matter. Well, it's fairly trivial, anyway. A professional trumpet player, for example, is required to know how to transpose music from one key to another, and on sight at that. This is because trumpet parts are usually written by the composers for trumpets keyed in whatever the tonic of the work in question happens to be -- thus a Symphony in D will call for trumpets in D. The problem this poses to the modern trumpet player is that few trumpeters today actually own trumpets in all the keys that might be required by the composer. Thus the trumpeter must transpose his or her parts on the spot, literally playing a note on the trumpet that is different from the note on the page, in order to actually produce the tone the composer requires.
(For those who are totally baffled by this, certain instruments aren't actually pitched in C, which is to say, that when the instrument's tonic note is played -- what is considered to be C on that instrument, with no valves pressed or keys closed -- the instrument is actually sounding some other note, as struck on the piano. On the trumpet as played by most high school and college trumpeters, this is B-flat. Thus, the parts that are printed for the trumpets will actually display a different key signature than the parts of the same work printed for, say, the flutes, which are pitched in C. Most professional orchestral trumpeters actually use trumpets that are pitched in C, but this does not mean they don't have to transpose.)
So of course, it wouldn't be too much of a musical problem to transpose Mozart's Symphony No. 40 from G-minor to E-minor; all it would take is someone to do the transposing. What I was wondering was if one could come up with a computer doohickey to do it for us, so one might stick a CD of the symphony in and have it play in E-minor as opposed to G-minor.
Which brings me to another question: do CDs reproduce pitch exactly, when played? When I listen to a disc of Beethoven's Eroica, are those opening chords actually E-flat major chords, or am I hearing them as merely major chords that may or may not have been reproduced specifically as E-flat major chords?