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If you're really desperate to get it before the A Class exam, you have to mess with the save files directly. The .rxdata files are a tad more intimidating to read (because unlike a .json, they're not really made to be actually read by people IIRC), but editing the species and item in there...
Ran into a similar issue. What I did was export a team with only one member in the S-Rank Manor, opened up the .json of the team, and then changed the Pokemon to Greninja, so that when I'd import it back, it'd be back as a Greninja.
Technically you can modify a lot of stuff about the Pokemon...