Please know that I'm responding to you honestly, but in a tone that may sound "harsh" (sorry) - the fact that you have not been able to "hear" that this is a 12 bar Blues in Bb (Harmonically), (yes, with, at times, a more complex Melody), is evidence to me that you need to (continue?) to listen to this recording many dozens of times, singing the roots of the basic Blues changes, playing the roots of those changes, and NOT worry, AT ALL, about "Understand(ing) theoretically the changes". Reading Theoretical Analysis of a 12 bar Blues in Bb, will do little to help you HEAR the Harmony, and the Bass notes on the recording that are (generally) very supportive of the Harmonic structure.
I've listened to this tune maybe a dozen times in the past few days (Thanks!) and can confirm that they stay in the 12 bar Blues structure, Harmonically speaking, for the whole track. (Yes, there are some very sophisticated Harmonic&Melodic devices at play on the recording.)
Also - Good for you for asking your question - asking for help is a good sign.
Q: Do you know the basic Harmonic form of a 12 bar Blues in Bb? (I'm not sure if you do or not...sorry!)
IMFO, of course.
Thanks.