Fair enough, Yoc & John.
What I had in mind was not so much a 'cosmetic' improvement (that may or may not actually be considered an improvement by some), but correction of glaring defects that most readers would consider desireable (i.e. increasing contrast on a washed-out, hard-to-read page; correcting blurred images & text, if possible)
I agree that original scanners should be consulted before tampering with their work. But, looking to the future when original scanners may no longer be around, it will be up to the 'Keepers of the Flame' on this & other such sites to determine if a submitted 'improved' file is worth keeping on the site.