Today, traditional OS SKU migration involves creating a new node, cordoning and draining existing nodes, and then deleting existing nodes. This can involve a large surge of core count as new nodes are added, as well as manual intervention to cordon and drain.
The OS SKU in-place migration feature, now in public preview, allows you to trigger a node image upgrade between one Linux SKU (i.e. Ubuntu) to another (i.e. Azure Linux) on an existing nodepool.
To learn more, visit: https://aka.ms/aks/OSSKUMigration