When a new Roman emperor was crowned, it's clear that the coins from the old emperor remained in circulation. So I expect a Roman citizen in Trajan's time to pay without issues with this Denarius Especially but not exclusively in remote parts of the Empire, where "new" coins arrived later than in Rome. Question is how long. Was there any rule? Or the average Joe didn't care about the emperor and the reverse and just knew that when he needs to pay something with 4 Denarii, he will take 4 silver coins about the same size from his pocket? Same thing with 3 sestertii, he would need to pay with 3 bulky bronzes, no matter what was written on them. When I started collecting, I was amazed about the large number of varieties for all coins. I don't think an average citizen was too interested about legends or even a middle class Roman had the chance to see all the coins with the portrait of the current emperor. In the time of, let's say, Antoninus Pius, was one like this still in circulation?