What counts as a purely mechanical adjustment? Can you apply any trivial and reversible operation to the notation? I guess changing the time signature to an equivalent one is acceptable. Changing all the notes to half their length? Replacing dotted notes by ligatures? Switching the key would be probably a "creative" change, but it's just as mechanical as the others. What if I move the key up a semitone but then I add a text that says to tune the instrument a semitone down? Does said text really matter, considering the historical tuning has changed by almost a whole semitone along the centuries?
I feel very uneasy changing the notation of the original, even if the music sounds the same, and naming it "urtext".
No big deal... after all, you can buy "urtext" with editor-provided fingerings from many publishers, so the word is almost meaningless at this point.
Any change that can be made without "skill and judgment" as would be involved in the creation of an original work is definitively mechanical per Canadian copyright law. Changing the clef from one to another is just like changing the font from one to another, which in Canada is considered the prototypical mechanical change.
Maybe you don't like modern clefs, but by no means are they any sort of "original" work by the standards of copyright law in any country. (Even in the small handful of countries that recognize typographical arrangements/engravings per se as copyrightable in themselves, like the UK, changing the clefs used is not considered original in any way.)
Pure transposition, by the way, is also not considered an original copyrightable creative change.
I think you have focused in on the wrong aspect entirely from the musical standpoint... the purpose of any of these editions is not to be a facsimile of some existing document (facsimiles of autograph manuscripts exist), but really just to be critical editions taking into account a number of sources and intending to establish the work as it is in a certain state (generally, an early state) according to certain editorial principles, rather than reflecting the editor's personal creative opinions on how the work should be interpreted.
As for the fingerings provided in some critical editions, these are not really relevant, because they are clearly marked (when applicable) as separate from the text itself, and they can easily be redacted (as we need to do sometimes) or ignored. By the same token, the preface or editorial footnotes in these editions are separate from the presented musical object — do you believe a preface or footnotes somehow makes the thing provided in the same pages indescribable?
Thanks for your detailed answer! I guess I'm just used to critical editions in poetry, which are supposed to be identical to a facsimile but without transparencies nor coffee stains. The critical comments and the text are separated in odd/even pages. The even pages have a lot of notes, and are sometimes longer than the actual text! They explain the history of a single comma along the several editions of the poem in question, etc. I always hoped that musical urtexts would be like that, but it doesn't seem to be the case. Fortunately, we have IMSLP where we can at least see most of the originals nowadays!
I feel very uneasy changing the notation of the original, even if the music sounds the same, and naming it "urtext".
No big deal... after all, you can buy "urtext" with editor-provided fingerings from many publishers, so the word is almost meaningless at this point.