I think that aslan didnt change from a lion but turned into something more glorified. Still a lion but mabye more heavenly! like jesus in the transfigureation... just a guess

I think Lewis made Aslan a Lion, because lions are often considered royal and mighty, like Jesus Christ. There also comes in the part about the Lamb Aslan appeared as at the end of VDT. Lambs are gentle, and Aslan, in spite of his power, is truly gentle and loving towards the loyal Narnians.

In the end, Jesus Christ is the Lion and the Lamb due to the described nature above of both creatures, and thus fits Aslan perfectly.

Did you notice that this is the first time in all the books that Lewis refers to Aslan as He with a capital H?

I think it emphasizes that Aslan turns into something greater than the form he had in Narnia, now that he is (and they all finally are) in his own country.

Excellent points! For this reason, and given the context (that the new Narnia, along with other parts, like the new England, is a part of heaven), my interpretation is that Aslan's form changes to that of Jesus - but more than the man he was on earth, but the glorified form he has in heaven.

Did you notice that this is the first time in all the books that Lewis refers to Aslan as He with a capital H?

Really? Now that is interesting, and probably the biggest clue we shall ever get as to what form Aslan was taking. Occasionally I capitalize a pronoun representing Aslan whenever I'm trying to make a biblical parallel... Now I feel slightly more vindicated in doing so.

Did you notice that this is the first time in all the books that Lewis refers to Aslan as He with a capital H?

I think it emphasizes that Aslan turns into something greater than the form he had in Narnia, now that he is (and they all finally are) in his own country.

Excellent points! For this reason, and given the context (that the new Narnia, along with other parts, like the new England, is a part of heaven), my interpretation is that Aslan's form changes to that of Jesus - but more than the man he was on earth, but the glorified form he has in heaven.

I share that interpretation. It would be in his resurrected body, even as it is after the Ascension, at the right hand of the Father.

So also is the resurrection of the dead. It is sown in corruption; it is raised in incorruption:
It is sown in dishonour; it is raised in glory: it is sown in weakness; it is raised in power:
It is sown a natural body; it is raised a spiritual body. There is a natural body, and there is a spiritual body.

The picture that always comes to my mind is that Aslan goes from looking like a Lion to looking like the Lion of the Tribe of Judah (and I think that is part of the reason that Lewis chose a Lion to be the Christ figure in Narnia - that and that lions are referred to as "king of the beasts"