ramifications of tweaks to subject line

[Gary Aitken]

Can someone summarize the ramifications of tweaking the subject line 
in a post?

Assume:
  Some-original-subject
Normal reply yields:
  Re: Some-original-subject

If a reply is made and the subject line is (mistake, whatever) trashed,
then manually re-inserted, does
  re: Some-original-subject
(change in case of Re) preserve threading?
Does change in case of words in subject line preserve threading?
More generally, what permutations of the original text still preserve
threading -- addition/deletion of various kinds of white-space, etc.
Is threading solely in the domain of the mail reader, or is there 
something in the header besides the text of the header itself that
aids threading?

If a subject digresses and one changes the subject line to
  Re: New-subject (was: Some-original-subject)
is threading preserved?

Thanks,

Gary