Poultry was once not considered fleishig.
If someone had eaten chicken immediately before the rabbis decreed that poultry was fleishig, would he have had to wait his customary number of hours before drinking milk?
I think there is some confusion. A rephrase of the question:
Reuben eats chicken and plans to eat milk during the same meal. After he has a bite of chicken but before he drinks the milk, the Rabbis decree that chicken is to be considered as meat. Since the chicken was not meat when he ate it, may Reuben drink the milk or does the decree affect the past eating?