Can you really change the password to a mac on reboot without knowing the old one? It depends on the encryption method.
For example, a non-System drive (i.e. A hard drive that doesn't have the Operating System on it) encrypted with Truecrypt will be completely unaffected by an OS reinstall.
Ideally, you would copy data off to a safe temporary location, reinstall, and copy the data back, then wipe the temporary area clean.
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