Did God change, or did sin?
Well, you know, God could see how things were back in the thirties. Everyone was so poor they couldn't even afford color. Look at all those old photos everyone says are B&W. They're not, they were taken with color film but everyone was so poor they just had to go without color. God knew people would spend all of what little cash they had on contraceptives and then be hungry, so in His infinite mercy and wisdom, He just said that for the time being it was wrong.
Now, He knows things are better and everyone is more educated as well as more sophisticated due to how easy it is to be informed, so sure, God changed. But He didn't really change, H just waited until we were better off and had the free time to really enjoy contraception then opened the eyes of all those people who interpret the Word for themselves and showed them that contraception is just fine.
Medical knowledge changed in that we now know that Fathers are not the generators of life, as was the understanding until the mid-1800’s. Contraception does not necessarily kill. Sterilization and barrier methods destroy no more lives than celibacy or NFP.