"Operation Undercover" was done between September 1971 and September 1972. During the project, the open-air shopping concourse was enclosed and climate-controlled.
Graphic from Merle Hay Plaza, Incorporated

In this vintage view, we see a newly-roofed North Wing and (now indoor) storefront of its S.S. Kresge dime store. As part of the roofing renovation, the shopping complex was renamed MERLE HAY MALL.
Photo from The Des Moines Register

Soon after the mall was enclosed, it was expanded with a new West Wing. This included a bi-level section, known as the Bridge Court. The centerpiece of this area was the controversial "UpDown" (a.k.a. "Trike Angel") sculpture, seen in the foreground. It was created by Mark Jacobsen.
Photo from The Des Moines Register

Younkers Store for Homes was part of the early '70s mall expansion and renovation.
Drawing from http://www.dmoed.org

Younkers' main MERLE HAY store had been enlarged in 1963 (in medium gray). A second enlargement (in lighter gray) was completed at a later date. Sears' was also expanded. The mall proper was enlarged as part of the construction of the West Wing (in light gray). This project got underway in 1973. With the completion of all modifications, in October 1974, the complex encompassed around 1.2 million leasable square feet and housed 137 stores and services.