We are what has changed. A mere mortal even to this day cannot enter heaven. In order for a person to enter into heaven they must become something more. Just as we believe Jesus is "true God, and true man", so we believe that the only thing that makes it possible for us to enter into heaven is to also become a composite of humanity and divinity.
Scripture describes this composite in a variety of metaphors. We are grafted onto Jesus so that he is the vine and we are the branches or we can use the image of a body where we are other parts and he is the head. Stated in the negative "apart from me you can do nothing." More particularly, apart from him we cannot enter heaven.
Today we celebrate Jesus showing us the way to eternal life in heaven. When he ascended he did not leave his humanity behind, he ascended body and soul, humanity and divinity. He modeled for a us what we must do. This is why he also insisted that we must remain in him. It is "through him, with him and in him" that our fragile humanity enter into that eternal kingdom of heaven.
As we walk through our day to day existence do we really consciously strive remain in him? We are so easily distracted, preoccupied. We can often forget about him completely. The good news is that he never forgets about us. On this day of the Lord, the Sunday of the Ascension, no matter what happens let us practice remaining in him.