[Just_Smith]

As you said - that's ideal. The home people trade-up to is usually more expensive than the home they move from (not counting empty-nesters) and - ideally - there will be a myriad of factors that can help one finish paying off their new more-expensive home within the same timespan, like more income or selling at a profit. But those aren't necessarily givens, as they're determined by things like your locale and the state of the housing market.