Paradise Pier is planning on not only adding charm and feel (through architecture and details ala Main St and NOS), but also re-branding as a "destination" for the whole family. The huge Mickey head and SSS practically shout at you "Hey! It's fun over here!" The old metallic sun scared people away.
Architecture alone will not rebrand an area as a destination for the entire family. Look at World Showcase at Epcot. For the longest time (perhaps still to this day) it was virtually impossible to get a family with teenage or younger children to go over there, other than perhaps to watch Illuminations at night. It's one of the most architecturally diverse places on earth, with cultural wonders to explore, things to discover, yet for the longest time it was not very popular amongst a wide age range.
So it's not necessarily that they needed the Mickey head on the Fun Wheel in order to fix things, but it's a giant, in-your-face way of announcing to everyone that the far side of the pier is a place you want to check out.
The Main St. USA style land is going to be built: Buena Vista Street. DCA needs a Fantasyland equivalent, and the Pier is going to do that. The Mickey Face is a quick and dirty way of getting the message across about what the Pier is all about, especially to the kinds of demographics (families = $) that DCA is trying to increase its appeal towards.
They didn't "switch them." Both the scary sun icon and the modernistic, almost abstract Mickey on Screamin were both removed. With a blank canvas to work with, it made far better sense to re-brand the Fun Wheel as Mickey's Fun Wheel, and the Screamin loop was a much better fit for a single-circle shaped object. It wasn't a switch - it was removing two icons that had become theme-inappropriate under the "new Pier" look, and therefore they had two empty spaces to work with, and chose the most appropriate locations for the sunburst and a Mickey face.But then why switch them? Is there a decision matrix that determined a sun rollercoaster and a Mickey Ferriswheel do better than a Mickey rollercoaster and a sun ferris wheel? It confuses me.