Another thread inspired plotbunnie, so beware.

Lilly worked with the DoM (Cliche' I know) and had come up with a spell while in seclusion that would mimic the Phoenix's rebirth. unsure if it works James allows it to be cast on him to test it.

Voldimort comes for harry, he battles James. Voldimort hits him with an AK But he's not killed instantly because of the charm. He hears Voldimort talking to Lilly, and Lilly begging for Harry's life. Moving as quickly as he can he trys to get outside to call for help he makes it out the door only to hear a high shrill voice scream out the killing curse. this time he dies, but the Phoenix charm and the previous Killing curse linked Him and Tom. Tom's body is consumed (setting the house on fire in the process) and James is reborn but his new body Has a cut on the forehead where the second curse struck him, and he has no memories. he is effectively a newborn. Lilly, not knowing this, takes the real harry and Disapperates thinking her husband dead and the charm a failure. Sirius and Hagrid show up minutes later and the story continues down whatever thread from there.



Challenge: James (as Harry) Has a new Animagus form. Phoenix. There are limits on the form though.
He has Eternal Rebirth, but only keeps his knowledge as a Phoenix. If he's killed while he's a Human he starts all over again, and there will be no way to get his former memories back (Except perhaps from Voldimort's mind.)
When he Sings the effects of the song are equal to the emotions he's feeling at the moment. This means that Unless he's got MPD He's not going to be depressed one minute and cheering people up with phoenix song the next.
His Tears have No Healing Properties Unless shed for someone he genuinely loves. This works indirectly though. If he knew a loved one would be hurt by someone elses death he can shed healing tears for them, but it causes a great deal of stress to his phoenix body, and if he transforms back too soon after doing so the strain trainsfers to his human body as well.
When Lifting heavy loads he strains his magic Proportional to the weight of the load he's lifting.
When in Phoenix form His Instinct is to protect the Innocent, It's where he gets his 'Saving people thing' from. however when he's transformed his subconcious controls the reflex, meaning that as a phoenix if he see's a harmful spell he Automatically appears in front of it.

 

But where would it lead? All you have here is a plot device, and the miss-spelling of Voldemort and Lily.

Perhaps in fifth year, when 'Harry' is in Voldemort's head, he chances upon his memory of the attack on Godric's Hollow. He realises his life has been a lie, and searches out for his forgotten memories.

Also, what's Lily been doing with Harry throughout this time? I doubt that Lily, as an unspeakable, would allow her son to be unprepared for the eventual conflict with Voldemort. Unless, of course, James was also born 'as the seventh month dies'. I don't remember if JKR gave him a DoB, so you could satisfy the prophecy that way...

 

Well, I see one flaw; if Lily and the true Harry are still alive, wouldn't they remark that there is an extra baby? Wouldn't she wonder why there is two Harry Potter, one next to her, and another living with her awful sister?

It means either the author has to make a whole alternate story, re-doing the first, second, third, fourth and fifth year again... which due to most authors uncreative mind, would lead probably to some very cliché fanfiction.

Anyway, Harry being James, hum...
[Jbern fanboy mode ON]
The Lie I've Lived.
[Jbern fanboy mode OFF]

 

I know about the obvious Plot flaws. I'm not likely to write the story though. Best I can come up with on the fly is that Either Lilly fled the country with harry and therefore has no reason to know that 'harry' is making peridic appearences, or she assumes that Dumbledore has set up a decoy and is waiting for him to contact her with the 'All's clear'.

if I were to write it I'd probably start out with lilly coming back for dumbledore's funeral and actually introducing herself to James (Under her OotP codename) only to be shocked that the double appearant has no knowledge of her and her son.
James (As harry) starts re-gaining his memory after a brush with Toms last horcrux shatters the phoenix charm giving him back his former animagus form and such. Course That'd probably be just as the real harry gives him the finishing blow.