They recently moved the location by Park Meadows Mall to a closed Sports Authority location in Greenwood Village because that was a bigger store. The old location was an inline big box strip like Belle Isle, it was cramped and they needed more space. All in this area but the one in Downtown Denver are typical big box stores. The downtown "Flagship" location right off I-25 at Confluence Park was the 1901 Denver Tramway Power Company Building they remodeled and received $6.3 million in subsidies to open up there.

Here's a page from the architect on the project.
Mithun - REI Denver Flagship Store