Well, it's official. The USCCB has voted to allow parishes to begin using the sung parts of the Mass employing the new translation early - beginning in September, if the local bishop permits. Here is the statement. Kudos to the bishops for realizing that Advent is rather a poor time of year to begin singing new settings. Sentiment around the web (on Facebook and the blogs) has been mixed, but mostly positive.
I have been in consultation with our parish music director - and after an experience last fall where we attended a choral reading session with both new Mass settings and re-worked familiar ones, we are in agreement that new, at least for now, is better. Somehow, music and words get entrenched in our brains together, so that changing words to familiar tunes (and adding extra notes to accommodate the extra words) seems to make it much more difficult. We, as trained musicians and pretty good sight readers normally, found it very hard to put new words to old music. Maybe later, when we learn the new words well, it will be easier to go back and resurrect old settings.
So, with the opportunity to introduce the new sung parts more gradually, and the spoken parts beginning in Advent, parishes should have an easier time of it. The bishops' decision is one of great common sense.