Although 1930 isn’t technically part of the 1920s, stylistically jazz was on the cusp of changing. 1920s forms were mostly blues or multi-strained (as in New Orleans march-based forms). Around 1930 Louis Armstrong and others started to shift from the multi-strained forms to the more modern pop song chorus forms like aaba and abac. A few 16-bar tunes survived, but mostly, 32 bars became the norm. The earlier multi-strained pieces often had 16 or 32-bar trios which were usually repeated for solos. Tiger Rag is a prime example. By 1930 the shorter strains started to be eliminated or replaced by interludes and modulations. This is the situation for Old Man Blues.
The title gives away Ellington’s initial source: Jerome Kern and Oscar Hammerstein II’s Ol’ Man River (which became Old Man River) from the 1927 Broadway smash Showboat. This song became iconic almost immediately and nearly a century later remains one of the greatest songs ever written. The words and the story they tell sets the tempo at a slow pace for maximum emotional effect. Ellington’s interest is purely in the chord progression to be reset as an uptempo barnburner.
Jazz musicians are quite familiar with contrafacts (new melodies written over the chord progressions of older songs), most notably, The Blues and I Got Rhythm. The beboppers were notorious for this and expanded into a number of other standard chord progressions, like Honeysuckle Rose, How High The Moon, and Back Home Again In Indiana. Actually, this practice began decades earlier in New Orleans primarily with The Blues and Tiger Rag (Bill Bailey and Milenberg Joys) and expanded throughout the 1930s.
Normally jazz composers will enhance the chords of standards by adding or altering tensions to the pre-existing chords, adding passing chords, or in some cases substituting different chords altogether. Ever the creative musician, Ellington uses all three techniques to transform Old Man River into Old Man Blues.