When she finished the album, "Birthday" was scheduled second.
Pre-"Roar":
1. Roar
2. Birthday
3. Unconditionally
4. Walking On Air (like E.T.)
5. This Is How We Do
6. Legendary Lovers
7. By The Grace of God or This Moment
Then, midway into "Roar," she decided to release "Unconditionally" second because it was not only one of her favorites, but it was also really well received at the listening parties. Most importantly, it was the program director at Z100's favorite. Plus, she knew she was doing something in conjunction with Unicef, and I'm not sure how that figured in, but somehow it did.
At that point (sometime in mid-September), "Birthday" was moved from second to fourth. "Walking On Air" was moved from fourth to third (SNL, GMA, Ellen, iHeart, iTunes, etc.)
Mid-"Roar," Pre-Prism:
1. Roar
2. Unconditionally
3. Walking On Air
4. Birthday
5. This Is How We Do
6. Legendary Lovers
7. This Moment or By The Grace of God
Initially, when "Dark Horse" was taking off on Rhythmic, they still wanted to release "Walking On Air" third. That's why the label wasn't even sure "Dark Horse" was going to be a single, despite the early airplay--everyone involved wanted "Walking On Air."
Katy was pissed. Despite Capitol going ahead and pushing it to rhythmic and pop the next week, Katy didn't consider it official until the end of the month--that's why she didn't shoot the video until January.
From December through March, "Birthday" was still set to follow, and it did. She didn't want to release the pre-releases back-to-back. "Walking On Air" was going to be the fifth single (hence why she never stopped performing it -- she performed it at that Japan event in early March, and yet again at a Radio1 festival in late April, right after "Birthday" was released).
Post-Dark Horse, Pre-May 2014
1. Roar
2. Unconditionally
3. Dark Horse
4. Birthday
5. Walking On Air
6. Legendary Lovers
7, This Moment or By The Grace of God
Since "Dark Horse" was rhythmic, and she expected "This Is How We Do" to be a big crossover summer hit at rhythmic, she yanked it once "Dark Horse" became official--she wanted all of the singles to sound different. It didn't become a contender again until EMI sent the song to Australia/New Zealand and pushed it there without Capitol or Katy's permission.
In May, Katy decided to release this last minute instead of "Walking On Air" -- the label was convinced it was a hit, Katy thought it was a hit (despite being OK with not releasing it post-"Dark Horse") and it was one of the very early crowd-favorites on tour. "Walking On Air" was axed.
Up until June, "Legendary Lovers" was supposed to follow, but then "Walking On Air" became a contender yet again when Capitol started hearing from music directors complaining about "This Is How We Do." Radio wanted "Birthday" and "Walking On Air" from the get go. She wasn't going to release both "Legendary Lovers" and "Walking On Air" -- it was going to be one or the other because she wanted to release a ballad last. Then, she thought she might only be able to release one more single. Briefly, she thought about just releasing "This Moment" or "It Takes Two" instead, but then went back to either "Legendary Lovers" or "Walking On Air."
Now there may or may not be another single from the Prism album. They haven't reached a consensus on which one to release, and they are trying to gauge whether there's even enough interest. The radio teams are still trying to convince stations to play this more, especially HAC. I guess they gave up on Rhythmic.