In the original avatar, apparently her 'signal' (for lack of a better term) was too weak to make the transfer. They would have to find some way to boost that signal strength to effect the transfer, imho.
I've thought quite a bit about this and I don't believe so. I think there is another part to such a direct transfer. What I find interesting is that Mo'at believed (as if fact) that such a transfer could be done. Perhaps again her faith. But how could this have been previously accomplished? I think Mo'at's words are the key. "She must pass through the eye of Eywa, and return
." And remember in the film that Neytiri immediately looked up with a certain concern but understanding with her expression. (My interpretation admittedly.)
Now I agree that Mo'at said that Grace was very weak. And then that "there wasn't enough time". But was Mo'at referring to a physical weakness IE damage to Grace's physical body as we are perhaps incorrectly assuming? Or was Mo'at actually referring to Grace's spirit? Thus I think there is a part of this transfer that we aren't "seeing", meaning understanding...
Here is that scene from the original registered script before it was edited. As a bonus, you'll discover a bit more to the scene. I wish they'd left it in the film...SCRIPT SCENE
TIME CUT -- AVATAR JAKE carries Grace's HUMAN BODY, lightly in his arms like a child. Jake walks through the crowd to the dais, followed by NORM, who carries GRACE'S AVATAR.
Look where we are, Grace.
Her eyes flutter open. She looks up wonderingly at the Mother Tree.
(with a wan smile)
I need to take some samples.
Mo'at directs them to lay both bodies among the roots on the altar-rock.
Mo'at touches Jake's shoulder and he steps back.
The Great Mother may choose to save all that she is --
Mo'at's hand indicates Grace's AVATAR --
-- in this body.
CU JAKE, realizing the enormity of what she's saying.
Is that possible?
Possible, yes. She must pass through the Eye of Eywa -- and return. But Jakesully -- she is very weak.
Jake kneels next to Grace, taking her tiny human hand in his avatar hand.
Hang on, they're gonna fix you up.
Grace is barely conscious. She grips his hand.
I -- always held back. But you gave them your heart. I'm proud of you, Jake.
Jake feels his throat close with emotion.
Grace's eyes blaze with intensity though her voice is faint.
Help them. You do whatever it takes. You hear me?
I will./SCRIPT SCENE
Now, I'm going to focus on Mo'at's "Possible, yes" answer.
So is it Eywa who determines to allow "the return"? I think there's an additional factor to this. I think... that the spirit entering Eywa HAS TO BE DETERMINED
to return. It's just a theory. But Jake was successful. Why? Because he was driven with a pure spirit to transfer himself into his Avatar. Thus his spiritual determination to "return". He wanted to be Omatikaya and with Neytiri. Not within Eywa.
Grace, in my opinion, did not. She was amazed that "She's real" (referring to being in direct contact with Eywa) in the film. Thus I have to wonder if Grace's nature in wanting to explore that connection and experience Eywa, somehow blocked/altered/prevented/determined the outcome that Grace's spirit did not "return", thus not transferred into Grace's avatar body.So...
I'm not thinking that her "signal strength" needs to be boosted to effect a transfer to her avatar body at a later time. I think it could simply be a combination of the transfer process (meaning the combined spiritual power of the Omatikaya clan with Eywa's blessing) with Grace's spiritual determination to transfer into her Avatar. Thus, I maintain that I think the plot option exists where Grace could somehow contact Jake, Neytiri or even Mo'at to perform the transfer ritual. This is still dependent, of course, on her avatar body being maintained by the avatar link system (to keep the avatar body active), and her avatar body "maintained" (water, nutrition, skin circulation, stimulation, sanitation, etc.) while in storage.
Again, I want to also re-point to Mo'at's "Possible, yes" reply. How would she know this if a transfer from one body to another hasn't been done? Or has it in legend? There may be another combination to this. That a Na'vi (perhaps a Tsahìk) has somehow connected with Eywa and managed to transfer their spirit into Eywa, and returned a short period later. Sci-Fi far-fetched perhaps. But it's still worth considering...