I'm not aware that MS have announced the Win8 downgrade rights yet, but one would hope that having shelled out for a valid Win8-Pro (is that what it's called now?) license, you would be entitled to downgrade it to Win7-Pro - assuming you have access to the media.
Typically MS would kill Win7 product sales once Win8 is released, making Win7-Pro upgrades a non-option, unless you have Win8-Pro downgrade rights available.
It's not going to cost MS anything, it does boost the Win8 sales figured, and it does reduce the WinXP numbers, so why would they waste resources making Win8-Pro upgrades inelligable for Win7-Pro downgrade rights?
I'm also assuming that this is what business is intending to do with new Win8-Pro desktop/laptop purchases - same as they did with Vista purchases downgraded to WinXP.
However, MS have done plenty of inexplicable things in the past, so anything is possible.