Yes, you are misunderstanding me.
I believe that it was God's intention from the start that the old covenant would be temporary. God intended it to point towards the new covenant in Christ Jesus. That doesn't mean God changed at all, but it does mean that He did some things for a limited time.
If I sell my present car and buy a different car, have I changed? No. If I plant geraniums in a pot one year, and impatiens in that pot the next year, have I changed? No. If I build one chair from maple, and the next from oak, have I changed? No. So when God does things one way at one time, and a different way at a different time, why do you think He changed?
The old covenant was not "improved"; it was replaced. Nobody can follow the old covenant now; it is impossible, and has been since 70 AD. That's what seems to be at the core of the confusion: the two covenants are distinct. Read Galatians 4:21-31. Hagar was not "improved" to become Sarah, and Ishmael was not "improved" to become Isaac.