twistjusty,
Thanks for posting. This is a good question, one the media generally disregards.
This is just the people I know, but it seems like the majority of 'move-up' buyers are families in the 35-55 demographic. Some of them have gotten new jobs and thus feel like they deserve a bigger house, other just have a growing family and feel like they need more space, better schools, or a better neighborhood.
But your question was really how do these people do it. First, moving up is slightly less hard than buying straight into that bigger house. Let's crunch a few numbers.
Assume that the 1998 start home cost $200,000 and the upgrade home cost $300,000. Both doubled in 'value' so the buyers have an extra $200,000 in equity. They have only owned for 9 years, so they have built little equity from mortgage payments. Let's also assume these guys never pulled equity out of their home. So just the equity from the boom gives them a 20% down payment on the upgrade house.
But they are doubling their total mortgage ($200,000 to $400,000). How does that work? They have probably received some raises to compensate for inflation over the years. Let's assume they did better and actually got a 4% annual raise. Their income has risen by about 35%. This increased income means in real terms, their mortgage is just 1.4 times as expensive as their original house.
So what you would expect is these upgrade buyers will own a little less of their house. This graph shows what's really happening
At the same time, the number of owners who have 100% equity (they actually own their house instead of the bank) is around 1/3. My understanding is that number has not moved very much. So retired people (and the ultra rich) own their homes, they are not moving out very often, and everyone else is dragging down the percent of total equity. Look at the graph above and you'll notice that 2/3rds of all home owners owe 50% of the total value of real estate in America.
I hope this answers your question.