Yes this was already happening 2 yrs ago with the introduction to the polymer notes...
The $20 bill can be considered the "work horse" of all denominations for notes. They are used more frequently, there are more prefix's and they wear faster.
When the switch to the poly 5's, 10's and 100's were done basically as soon as the denomination was handed into the bank it was to be replaced with a poly note.
But for the $20's they slowly were taking them out of circulation while still keeping the "best" of the paper issues to ensure a smooth transistion with few if any shortages.