The Phoenix as Jean Grey
...in the beginning

Jean Grey is the only one who can save her friends from certain death as they plummet in a shuttle to the earth while deadly radiation also threatens to kill them all.

She absorbs the knowledge of piloting the shuttle from it's pilot through the power of telepathy and will shield herself and the crew with her power of telekinesis used as a shield.

It is the only conceivable plan that will work but does she have the strength to maintain the power levels needed to do all this?

The answer is no.  The radiation is coming through.  She is dying, her body withering away but her spirit determined to live to save her friends.

She cries out in desperation and the call is heard...by the Phoenix Force.

The Phoenix will save her and her friends but she takes over Jean's life by replicating her body and mind.  To heal Jean's body will take a long time.  It is placed in a cocoon and into the sea to heal as the Phoenix deceives everyone into believing that she is the true Jean Grey. 

In a way, she is, because it copied everything about her down to her very emotions.  However there is a difference.  She has almost unlimited power.  And as the saying goes, "absolute power corrupts absolutely" and THIS Jean Grey would soon discover this truth firsthand.