I suspect that since Windows 10 shares a lot of the same architecture already found in Windows 7 and Windows 8/8.1, that the in-place "upgrade" is going to be more like doing a service pack update, rather than requiring a complete clean installation (which of course you can do, as well), but in theory, all your apps should still work as before. It used to be that in-place upgrades only kept Microsoft apps working, but all the others needed re-installing. Supposedly that's not the case this time around.
We'll see...
John
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