That's how many people are leaving. How many people are coming in?

There's plenty of land here in Wyoming. Houses, not so much. Finding someone to build a house or to do any work on anything is not easy. For that reason I don't think 400k could move here if they wanted to, at least not all at once. Some may not like the snow, cold weather half of the year and the wind east of the Rockies. Some of the people from the mountainous region of northern California may like it. Anyone moving here should be self sufficient and able to fix most things themselves. Some of this can be offset by getting to know all your neighbors and local community.

As a side note Netflix to my surprise released a decent albeit violent movie that shows some of this state called Wind River staring Jeremy Renner, Elizabeth Olsen, Jon Bernthal. (2017) Most of the mountain scenes were filmed in Utah. [1] I think it would be incredibly challenging to film the whole thing in the Wind River National Forest

[1] - https://www.netflix.com/title/80173524