Put it pretty blunt, and basically it was meant to be. If you send your device in for the 4G LTE upgrade it will be sent back to you. Motorola refuses to upgrade any deivces that are rooted, hence you must unroot your device before sending it in to obtain the upgrade.
Of course this was explained to a better degree by another forum mod in Motorola support, your Xoom must be returned to factory settings and software when Motorola receives its.
Source: Android Police via Motorola Support
