I don’t really see a reason why God can’t change, especially when taken from our limited point of view. We might philosophically surmise that God is somehow changeless in God’s infinity, but that infinity is unknowable. There’s no reason to think that God-as-we-perceive-It doesn’t change. There is even positive evidence in the Old Testament – God is dissuaded and cajoled and convinced. God seems to ponder and deliberate and sometimes makes surprising decisions.
I think part of why some people feel God must be changeless has to do with the idea of God’s promises – that God must be changeless if God’s promises are to be reliable. Here again, though, I depart. I expect human beings around me to keep promises. That’s kind of an ethical minimum – let your yes be yes and your no be no. Keep your commitments. Most of the time, I’m even successful at this. Honestly, I hope for much more from God. I hope for God to shatter God’s promises, raining the splinters of our petty expectations down on us. And God has a history of breaking promises in our favor. God promises a Messiah and God provides God’s self, far more than a mere Anointed One to take the Davidic throne or a new Elijah to trumpet the prophetic message for all the world. If God had kept that particular promise, would there have been a Jesus Christ? I doubt it.
In fact, I think that Christ changed God. Or, if you prefer, the experience of Incarnation changed God. We can talk theoretically about the pre-existent Word which was present at the beginning of creation, but that Word became flesh in the mess of history and experienced the limited life that we all share, from bookend to bookend, birth to death. There’s something about experience that changes a person, perhaps even a Person. Perhaps even God.
God’s concerns change over time as the OT progresses in its storyline, disjointed as it sometimes is. From a garden built for two to the descendents of Abraham, God’s concern expands, out to an eschatological New Jerusalem, and then the Kingdom of God, brought into being somehow by an execution which is not allowed to be the last word. With Moses God wasn’t shy about wiping out a whole generation of firstborn in a night, or raining fire, but with Christ God does nothing whatsoever to even alleviate his suffering on the cross, to the point of leaving him abandoned, of letting him cry out in agony to an unanswering sky.
I think we confuse an unchanging God with a God who’s love is unchanging – that is, undiminished over time. I would argue, even, that God’s love only increases over time, seemingly encompassing more and more of the world. I feel that I can trust God to love me, but I have no reason to think I can trust God not to change. Honestly, I prefer that situation (which might be the real reason I believe it to be true). Life is defined by change. That which ceases to change is dead, fixed, stagnant, inoperate. For there to be a living God, I think there has to be a changing God.