It's a pretty significant and expensive project.
But it's not the first time the park has undergone major construction.
Nine years after the park opened, over the winter of 1922-23, it underwent a huge expansion. The existing single decked grandstand (the upper deck didn't come for another 12 years) was sliced into three pieces. Two of the three pieces were moved and then new seats were added between the moved pieces. The field was also lowered and addition seats were added in front of the original grandstands. All of this increased the capacity from 18,000 to 31,000.
I saw a blog post on the site There Used to Be A Ballpark that did a beautiful job of showing and describing the process. They even have an article from Popular Mechanics that detailed what was done. It's fascinating stuff. If you have any interest in old ballparks or Wrigley Field, you need to check it out.