Are pre-existing Villagers affected by workstations?
After asking this question, Villagers from 1.13 that you have traded with, then upgraded to 1.14 will maintain their Profession and trades in 1.14. Any that you have not traded with, will lose theirs, and can be assigned new ones at will.
However, can these pre-existing "Professional" Villagers be affected by workstations?
Solution 1:
I tested this, and yes, they are affected by workstations. I tried several different types and they all gave the villager the appropriate profession.