A sidegrade is a term used with computer software to refer to transferring a piece of software to another operating system than that for which the software was originally licensed, without necessarily changing the version of the software.
The term is most commonly used by software manufacturers and retailers to describe whether or not payment is required to transfer one's current software license to a different operating system. Some manufacturers require payment for a sidegrade, some simply require authorization in some form, and some make sidegrades transparent by allowing a licensed software serial number to work on any supported operating system.
In cases where certain service providers make a change to customer's account that entails neither an upgrade (which by their standards would denote an increase in service and a proportionate increase in revenue) nor a downgrade, they have been known to use the term 'sidegrade' internally.
The word is a transformation of the word "upgrade," which normally implies a change in version number but not operating system.