The NORTHLAND and EASTLAND centers were enclosed and climate-controlled simultaneously. The 50 million dollar dual renovation of the properties commenced in July 1973. Both mall's emerged fully-enclosed in August 1975.
Graphic from the Dayton Hudson Corporation

A view of the mammoth mall soon after it made the change from outdoor to indoor. A new J.C. Penney may be seen in the lower right. This store, which encompassed 284,000 square feet, was one of the largest in the chain, then based in New York City.
Photo from Malls of America Blogspot