By change is meant our glorification, our change from this earthly, mortal nature to an heavenly one [1Co 15:38-49ff].

God does not have to have done a thing before for us to believe that he can and will if he has promised to do so. That is the whole idea of being absolutely trustworthy and faithful—we can be sure of anything he says because he will not lie about what he can or will do. Nevertheless, these are some reasons why we can be certain that God will indeed change us from mortals to immortals

God can do it.

We know this for certain because

1. God has done it before!
   1. God did it when he raised Christ from the dead. In fact, it is one of the ways it is stated in prophecy and in the gospel (i.e., the teachings of the New Testament), Christ is the first to rise from the dead and his resurrection guarantees the resurrection of all others as God as promised.
   2. The existence of angels is another proof that God can change us to spirit beings with a new type of body
2. Since God created us, then reason tells us He can
   1. If God indeed created us, then he can upgrade (or downgrade) us if he so chooses to.
      1. Since he created angels too, and angels are higher than us in nature, then we know that he can upgrade us to be like them
      2. Since God himself exists, and God is higher than even angels, then we know that he can upgrade us to be like him _(which he has done with Jesus)_.
      3. Nebuchadnezzar is proof that God can downgrade us if he so chose to.

God will do it.

We know this because

1. He has promised to (which you, of course, already know, for that is why you asked the question in the first place). Not only do we know that God can do it, we know that he will do it because he has said he will, and God never, ever goes back on His word, whatever it will take, He will get it done.
2. He has done and is doing everything to make sure it is done [Rom 8:28]
   1. In summary, there are limitations against our being glorified which must be addressed it we will be changed
   2. It is one of the benefits of having scriptures, they show us the challenges facing our change from earthly to heavenly, mortal to spirit, earthly to heavenly and how those issues are taking care of by God. If we understand these things, then we can be certain that God will indeed fulfil this purpose
   3. This, however, will require an appreciation of how the world is run from the spirit perspective (not from man’s limited, near completely blind, perspective)
   4. We must understand that there are fixed laws that guide the working of even the spirit realm, we cannot be changed in violation of those laws. Because of these issues, Jesus had to die and also go to heaven to be our advocate and also sit at God’sright hand.
3. God loves us. This is evident from price he has had to pay. God loves us and really wants us to be glorified. His love guarantees it all [Rom 5:9-10; 8:32]
4. Also, the price that God has had to pay—the suffering and death of His Son and the sufferings and in many cases, death of his beloved servants and friends—to ensure this happens is too high for him to back out. If God is rational and if He is not a wicked sadist, then he will certainly finish what he has started.