When Sunny Gets Blue

Sounds like a chromatic approach to the C minor. Dim and half dim are often used as chromatic passing chords. Same thing happens at the end of the next bar. The original changes for bars 5 and 6 would just walk the bass line down chromatically C# C B Bb A

But I’m no hardcore theory guy. So l’m also interested in what an expert would say.

Regarding the ii-V's just shifting key centers, that's exactly how you need to think of it when you're playing. There is no way you're going to analyze a chart on the fly and think, that's a iii-V/ii. Just think that's another ii-V a whole step up from the last ii-V. In fact, in the heat of the moment, I think even bigger picture - major, minor, dominant.

With the exception of about 4 chords, the whole tune is just a bunch of ii-V's like most standards. So I expect Simon's analysis to be far simpler than yours.

Check out Stitt's version. Absolute perfection. Many, many years ago, I memorized the whole thing and still use nuggets from it today.