It's true that polygamy was not condemned in the OT, and it even seems that maybe it could be necessary sometimes, for example to fulfill the command to raise up children in your deceased brother's name or whatever (if I understand that correctly). HOWEVER....
The truth is:
With the exception of KINGS, it wasn't common. And even in the cases of men of God who did, it wasn't even their desire or plan.
Look here:
Abraham: one wife........ even though he was very wealthy, and powerful, ONE WIFE, all the way into his old age. It was only when his one wife pushed her maid servant on him (in an effort to try make God's promise happen) did he have another wife. It's kinda remarkable and notable that he didn't before that (despite all his wealth and power, all the way into his old age) and was apparently very much a "one-woman man".
Isaac: one wife, Rebekah
Jacob: He only wanted one wife. He worked 7 years for that one woman, and was tricked into marrying another, and had to agree to work 7 more years to have the one he wanted. Then the other two were given by his two wives to compete with each other in bearing children. Which, actually worked out for God's plan to form the twelve tribes of Israel, and fulfill God's promise to make his grandfather Abraham into a great nation. But really and truly it was that one that he wanted, and worked 14 years total just for her.
Joseph: Wasn't it one wife?
Prophets: one wife (if any).
Since many pages of the OT are covering the lives of kings, it's easy to get the impression that polygamy was normal for men of God but it wasn't. I remember when I first got saved and started reading the OT I kinda got the impression that polygamy was common and normal for men of God (and it kinda made me entertain the idea !).
But it wasn't, except for kings. It's the people who wanted that, not God. He warned them that a king would take their money, their daughters, etc.... God wanted to be their king, but they wanted to be like the other nations, so he gave them what they wanted, a "king" modeled after the other nations, with the harem and the whole shebang.
Anyway...........
There is also a passage in the NT, in the character qualifications for an overseer/elder/pastor in 1 Tim 3. The whole context is godly character, which should be exemplified in a church leader. Interestingly, in this list of godly character traits, it also says "a man of one woman" (or "husband of one wife"). So, we have to acknowledge that it must be a good thing, to be a man of one wife. Darn!
(Just kidding.)
The OP here gives an example of a ungodly man and asks the question "How did this change for Christians?". I think the example given is really misleading and I really don't think much has changed, in the sense that we see men of God being or trying to be a "man of one woman" even in the OT and in the NT it's clearly written that this is a good thing.
Having said that, it can still happen. Example, muslim man has three wives and gets saved. I wouldn't tell him to divorce any of his wives and he has a duty to all of his children.