[Tactitus]

In the US, buyers of mortgages are required to honor the terms of the original contract (they are only permitted to adjust certain fees associated with servicing the loan). That said, some mortgages contain on-demand clauses that permit the mortgage holder to call the loan under certain conditions (most commonly, whenever the property is sold). On-demand clauses with no conditions (allowing the holder of the mortgage to call the loan at any time for any reason) are extremely rare for the simple reason that whenever interest rates went up, there'd be no reason for them not to call the loan and relend the money at the higher rate.

Mortgages are bought and sold all the time here. They're bundled into packets (called Collateralized Mortgage Obligations) and then sub-divided into pieces that offer varying degrees of risk and return and are then sold. CMOs trade freely and as interest rates flucuate, or the mortgages mature, default or are paid off the CMOs can be broken apart, rebundled and resold. Although mortgages change hands more often than ever before, homeowners are actually seeing less of this activity because mortgage payments are increasingly handled by mortgage servicing companies--middlemen who simply collect payments and forward them to the (current) owners of the mortgage (for a fee ofc). The actual owners of the mortgages are typically not mortgage companies at all, but simply investors who buy CMOs much like they'd buy corporate bonds. I have some CMOs in my retirement account so in theory I could own (part of) my own mortgage.

My wife works in an office that does CMO pricing (mostly for home and car loans). She can go on for hours about tranches and CMOs and whatnot. I think I'd kill myself if I worked there; but she seems to like it and the pay is pretty good.