Good observations everyone. I have noticed the same thing, but to see all the changes commented on in one place is very convincing. I personally think it has a lot to do with making the songbook acceptable to different cultures and languages. After all, some of the hymns in the US from the 1800's had a military theme, so didn't sound too much out of place in the WT heartland in the early part of the last century. But you take the same words, translate to Swahili and try to get an African audience to sing it... I'm sure there were some awkward moments.
On another related note, the song 'Shulamite Maiden' has had the words completely changed, though the tune remains in the new songbook. I'm sure it is because the words were a little too explicit, even though they came right from the Song of Solomon.