If the cross and resurrection changed things, is the God of the Old Testament still the same God?
Yes. Even though the life of Christ culminated at the cross and was then picked up in resurrection power, which makes things different, the God of the Old Testament is still the same God. The cross and resurrection change the situation, but not the identity of God.
Les explains that the life of Christ culminated at the cross and was then picked up in resurrection power, and that does make things different. Yet he emphasizes that it is still the same God. The cross and resurrection do not introduce a different deity or replace the God of the Old Testament; the same God remains, even as this new situation unfolds.