Thank you for your help, Cagey! I appreciate that!I've never heard the term before, but it seems to be used to refer to people who come back to an old job after some training, or are expected to replace someone else, acting in the manner the other person did.
Here is an example, taken from a newspaper. it's the story of a policeman who quit the job after 17 years and opened a trucking business. Five years later, he was asked to return to the police force, and did, at a lower rank than the one he had when he left.
Realising he could still make a difference and the security of the police was appealing, Ron sold his trucks and after a short retraining course at the Police Academy, he famously returned to the Homicide Squad in April 1994 - as a constable. "Some people didn't like re-appointees," he recalls. "They thought I should have gone back to driving divvy vans. The mentality was, 'He's a retread.' "Source: Homicide cop Ron Iddles prepares for Victorian Police Association job (Sydney Morning Herald-Mar 22, 2014)
This example is from a Australian paper, but I also found examples from American media, mainly relating to sports. I chose this one because it seemed the clearest example of what 'retread' meant in this context.