The Upgrade and Downgrade rights for a software model will not get auto-populated and you have to feed them in manually. Below is an example for both -
For example, you have licenses for the software model Microsoft Word 2010, but no licenses or entitlements for Word 2007. Discovery finds installations of Word 2007 being used in your organization. Rather than force users to uninstall all instances of this unlicensed version, you decide to count installations of Word 2007 against your Word 2010 license. To do this, you configure Word 2007 as a downgrade child in the Word 2010 Software Model record. A Software Model record is automatically created for Word 2007 which specifies Word 2010 as the upgrade parent.
If a software version has a downgrade child or an upgrade version that can be counted against the parent, the number of installs counted is restricted to the number of available rights of the parent. For example, Microsoft Word 2010 has a downgrade to Word 2007. Both versions have an active counter. Microsoft Word 2010 finds all entitled copies of Word 2007, and also takes out of compliance any installs from that downgrade counter until the available downgrade rights are used. However, if Microsoft Word 2010 only has 100 rights, then the maximum number of rights to be taken from the downgrade counter is 100.
A downgrade can be applied in 2 ways. Please follow this doc link for the same - https://docs.servicenow.com/bundle/london-it-service-management/page/product/asset-management/task/t_DowngradingALicense.html
Similarly, an upgrade can be applied in 2 ways, please follow this doc link to understand the same - https://docs.servicenow.com/bundle/london-it-service-management/page/product/asset-management/task/t_UpgradingALicense.html