This is not quite correct on one singular point. The texts cannot be changed, only the musical setting. You cannot change any words and the setting must conform to the text. The text must not be changed to conform to a setting, metrical or otherwise. Liturgium Authenticam made this perfectly clear in dealing with translations. Textual translations are supplied by competent ecclesial authority and musicians are not allowed to change them at all.
When LA came out, musicians probably noticed the result of this in published psalters with settings. Since there was not time to correct all the musical settings to conform to this situation, the flow of English in particular was affected since the musical settings were not changed . Editors just corrected the texts as supplied to them with the existing music composed on the faulty translations. Unfortunately that left some psalms in worse shape since the music composed was based on incorrect translation. For it to work properly both music and text must be adjusted. The options composers have is the one that is and always has been proper to them. And that is composing for a supplied text without changing it.
Although it was never allowed to change a text to fit a musical setting for liturgical portions, this did happens for a variety of reasons: lack of musical ability, ignorance, the need to force text metrically, etc., however it was never intended to be such and composers are required to work from the texts as presented without making any changes. Even if attempted in good faith, innocently, it should be corrected. Music directors are the ones most directly confronted and responsible for instituting this properly.
This has seeped into the sung ordinaries as well. Glorias in particular have become notorious as an example to changing text to conform to meter AND style. The question of altering the meaning has been rendered moot, since any group or individual is not allowed to do it. The never have been really, it was just something people did to make up for this lack of compositional talent to realize any musical setting of text.
Unfortunately the correction of this situation will take time.
Joe B