There's little chance that Jyn Erso is Captain Phasma. For starters, Felicity Jones is 5'3, Gwendoline Christie is 6'3. For second, Rogue ONe takes place over 30 years before the force awakens, odds are Phasma is much younger than 50 years old.
I'm guessing as well that pretty much everyone on the Rebel side of ROgue One dies at the end of the film, possibly ending with Jyn sending the transmission to Leia's ship.