One of my issues with the idea of setting old texts to new tunes is the suitability of the new melody with the lyrics. I've sung too many upbeat songs about the blood of Christ or repentance. Reverent theological issues should be sung reverently and appropriately. This does not mean in a stodgy or forced manner but with an appropriate weight and significance.
It is perhaps for this reason that many have trouble with rich theological texts sung to bebop style tunes that seem to undercut the permanence of the eternal truths expressed. Texts and tunes should appropriately reflect one another: joyful expressions of thanksgiving with joyous music; mournful repentance of sin with solemn and dark melodies; pardon received with thankful hopeful tunes; eternal truths with rooted and firm, timeless melodies.
Alice Parker addresses the suitability of this pairing in The Anatomy of Melody:
"Hymns suffer particularly from this failure to pair [text and tune] effectively. There seems to be a feeling that if the words are talking about something holy, that is enough. For the discriminating singer, that is certainly not true: a careless text can reduce even a wonderful melody to ruins. Choosing by syllable count (the meter of the hymns) is basic but much more important is the nature of the tune (its mood, function, voice, tone) and the comfort level of the combination…
"Hymnals are full of bad marriages, and our sensitivities get dulled by constant exposure to bad models…Try consulting the metrical index in the back of any hymnal and look up the text for the first example in any meter. Then try that text with each of the succeeding metrically matched melodies in turn. They should all fit, but you’ll find that few of them do…there’s a very real sense of belonging or not belonging. It’s amazing how few are really good combinations.
"When hymns are selected by text, the suitability of the tune is usually ignored…Both text and tune must have their own integrity and then graciously accommodate each other."