Turns out Spitfire had a specific idea in mind to get Rainbow Dash in the Wonderbolts.

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But then realize that Spitfire would probably end up burning all of that rainbow dash fat off and that would mean that Rainbow Dash would no longer be a wonderbolt.
@DeltsDash  
Idk, I shouldn’t be repeating it. Sorry “@mage24365”
@Athletic-Dashie  
That’s whatmage24365 just said, why are you repeating it?
@mage24365  
Well, look at it this way. Rainbow Dash will be a part of Spitfire which means that Spitfire will still be a wonderbolt but with Rainbow Dash as a part of Spitfire.
@thatoneguy  
But she’ll always be a part of the team. Or at least a part of one of its .
thatoneguy
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Unfortunately for Rainbow Dash, her time in the Wonderbolts would be limited to just a few days before a rather messy discharge.
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The End wasn't The End - Found a new home after the great exodus of 2012

The Fluffiest
And that’s also why her cutie mark changed from the earlier seasons. Headcanon accepted.