It’s unclear why a software modification would affect a hardware upgrade but it’s Motorola’s decision and they are the ones who are handling the upgrade via a fedex send back program.
There is some good news though, since the Motorola Xoom has an unlockable bootloader you can flash back the stock ROM and send your Xoom back. Motorola has not made the shipping (original) ROM available so you should back up your shipping ROM if you plan on rooting before the upgrade.
The date for the upgrade is unclear but make sure you’re ready when the time comes.
Source: Motorola Droid Forums