William Blake's lines of verse "Jerusalem", which appear in the "Preface" to his poem "Milton", were written c.1804 and first printed c.1808.
They also appear, but with the phrase "these dark Satanic mills" (present tense) altered to "those dark Satanic mills" (by implication from context, past tense) in the well-known hymn for which the first musical score was written by Hubert Parry for "Fight for Right", an organisation which sought to build support for the British state's war effort during WW1.
The lines had previously appeared in 1915 in The Spirit of Man, an anthology of poetry and philosophy collected by the poet laureate Robert Bridges, dedicated to the monarch of Britain and similarly intended to boost support for the war. In the edition I have, the title page states that the anthology was "made" in 1915; the date of the "First Edition with India Paper" is given as January 1916; and no earlier edition printed on any other kind of paper is mentioned.
It is known that it was Bridges who approached Parry and asked him to set the lines to music, that Parry wrote the music on 10 March 1916, and that the song was first performed publicly on 28 March 1916.
I was expecting Bridges's anthology to contain the altered version of Blake's words. But in fact it doesn't. It has the words that Blake actually wrote: "these dark Satanic mills".
Prior to the 28 March 1916 performance, although it could not have been before 10 March, the Curwen publishing house published sheet music containing both Parry's tune and the words for singing along to it. This had the altered word "those", not Blake's original "these", so the first appearance of the alteration was certainly no later than 1916 - for example it was not as late as Edward Elgar's rescoring of Parry's music for orchestra in 1922.
Is there a record of who changed the words (or "asked" Parry to), when exactly they did it and in what exact circumstances, what aims they expressed, or any discussions they had about making the alteration?