A sidegrade is a term used with computer software

Computer software

Computer software, or just software, is a collection of computer programs and related data that provide the instructions for telling a computer what to do and how to do it....

 to refer to transferring a piece of software to another operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer 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

Upgrade

The term upgrade refers to the replacement of a product with a newer version of the same product. It is most often used in computing and consumer electronics, generally meaning a replacement of hardware, software or firmware with a newer or better version, in order to bring the system up to date...

," which normally implies a change in version number but not operating system.