You can request for substitution at three different times of the year. The portal for that is currently closed, so I may be off on dates. However, I think one is sometime in summer prior to summer convention, one is prior to January 10 (they meet during MLK day), and the other is prior to February 1 (they meet during TMEA). You need to be able to submit a pdf of the score and a recording. You also need justification as to why you want to select this piece (and that needs to be more than "it will be a good fit for my ensemble.").
I've requested for substitution at least 4 times and have always been granted permission. It's a simple process.