http://austin.towers.net/one-way-out...o-way-traffic/
So it looks like they're converting 5th (from !-35 to Brazos) and Colorado (from Cesar Chavez to 9th) to two-way streets.
As the article says, the Colorado change makes sense as buildings on either side densify and the entertainment districts get more popular. The 5th St. conversion is part of the pre-work that will allow the downtown rail station expansion to move forward. Because 4th will be cut off from all vehicle traffic from I-35 to Trinity (to accommodate more rail), 5th needs to be converted to handle all incoming and outgoing traffic.