Hi Francesco Landoni ,
Welcome to the Microsoft Q&A Platform! Thank you for asking your question here.
To enable long-term support (LTS) for Kubernetes version 1.27.9, you'll need to move your cluster to the Premium tier and enable the AKSLongTermSupport plan. The pricing for LTS is tied to the Premium tier, so you'll be charged accordingly.
Please refer to these for a better understanding Long-term support for Azure Kubernetes Service (AKS) versions.
If you don't update your cluster, you may not receive security patches and updates, which can leave your cluster vulnerable to security risks. Additionally, some add-ons and features may not work as expected or may become unsupported.
To ensure security and access to critical updates, it’s essential to keep your AKS clusters up to date with supported Kubernetes versions.
If you have any further queries, do let us know.
If the answer is helpful, please click "Accept Answer" and "Upvote it."