I was in a similar situation... I was originally DCA based on 37 but awarded ORD during a vacancy in training. I emailed pilot planning and asked about it. They didn't give me an activation date but they just said that activation is done based on company need and seniority. I was fortunate and was activated the very first month of the award. (Vacancy award for 0116 and was activated for 0116) so I only had to do one month in DCA. This isn't always the case though. There were people in my Indoc class that were on the bus who were awarded ORD in the same vacancy I was but have not been activated yet. Same goes for people I know in the 73 in IAH.