The part that would be "lost" would be the segment from the "Old River Control Structure" to New Orleans.
The reason is that the Atchafalaya River is deeper and provides a MUCH shorter path to the Gulf of Mexico. The Mississippi has been "wanting" to cut a channel from its existing bed to the Atchafalaya for decades. The reason the "Old River Control Structure" was built was to prevent that.
In the high water of 1973, the Mississippi almost undercut the then-existing control structure, and to protect that structure was the reason the Morganza Floodway was opened that year.
Since that time, the original control structure has been re-inforced, and supplemented by the "Low Sill Structure"...a separate additional control structure to provide more bypass capacity.
If the Mississippi "does" manage to switch beds, salt water will intrude back up the now "slack flow" current channel, and communities (and industries) that take their fresh water supplies from the river would have to be drastically re-designed, or abandoned.
Search term: "Old River Control Structure" will take you to lots of info.
Thanks for the explanation. It’s fascinating stuff. I knew that the Army CoEs spent a lot of effort trying to control the flooding along the river, but I never knew the river is threatening to do this. Incredible!