
I can't say for sure with the IIvx -> C650 upgrade, but I wondered the
same thing when I ordered my LC -> LC III upgrade.  Turns out the "upgrade"
is actually an entire CPU minus any disk drives.  You pull the floppy and
hard drives out of the old one, stick them in the new one, and you've got
an LC III.

The IIvx -> C650 may be the same thing.

It might be something to look into for those people who are unhappy that
Apple only sells Macs pre-packaged with the drives.  Of course, the price
is quite a bit higher without the trade-in...