How Often Are Presidential Limousines "traded In". How Badly Does One Of Them Bad Boys Depreciate?

I had once heard that they up-grade every year. The president can't be running around in "old" wheels so he always has a brand new vehicle. Presumably, right now it's a 2009 and, by the end of the year, it'll be traded for a 2010.
I mean, gee, there are people unemployed and hungry but we can't have our leader in a 2008 vehicle, can we?
I tell myself that it's necessity, they have to constantly upgrade the security devices so it makes sense to change vehicles but, still… there are old ladies in pain because they can't afford their meds, homeless disabled vets living in the streets, so many people living in poverty that a part of me feels bad about the expense…

They are not traded in. They are dismantled. There is only one company who builds them for the president. They don't want their secrets to get out so they take them apart. And since technology is changing every day their are many upgrades to be done on the new ones.