You can now upgrade Ubuntu Server Virtual Machine Scale Sets on Azure to Ubuntu Pro without rebuilding the set. The scale set keeps the same Ubuntu LTS image, and the change is a license update.
This works with Canonical Ubuntu 18.04 LTS and later on Azure Marketplace. Custom images, including golden images and copies in Azure Compute Gallery, are unsupported.
Details and limits: Ubuntu Pro in-place upgrades for Virtual Machine Scale Sets on Azure. Azure CLI for Uniform and Flexible orchestration: How to upgrade Virtual Machine Scale Sets to Ubuntu Pro.
If you have converted a scale set, share the orchestration mode, Ubuntu release, and instance count.