I'm not a lawyer, but the year in a copyright notice is supposed to be the year of original publication. If you have a document first published in 2012 with a "Copyright 2012" notice and you change the year to 2014 without making any other changes, the original publication year is still 2012 but now the copyright notice is incorrect, which makes it legally equivalent to no notice at all (although copyright notices have little legal meaning anyway in Berne Convention countries). See here, for example:
It makes sense to bump the user-visible copyright notices of lilypond, convert-ly, and the like to whatever year those tools were last substantively changed, and to update the notices in individual files to reflect when they were last changed, but a global search-and-replace is probably a bad idea.
-- Ben