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I'm personally thinking it isn't possible. I'm going to direct you to these wiki articles on PCI and PCIe - explains the differences and even shows the pinouts. http://en.wikipedia.org/wiki/Conventional_PCI http://en.wikipedia.org/wiki/PCIE
One major obstacle I foresee is getting a card to fit in a case slot after crosswiring a bridge. After I compare pinouts, that's minor.
Why would you want to do this? You can't mod the actual slot however an adapter card could work if the device you want to use has a low profile bracket.
Probably for a graphics card or something. All graphics cards have been speaking PCIe since it was introduced in 2004. Any card made for AGP/PCI uses a bridge chip which speaks PCIe on one end and AGP/PCI on the other