Hi Mohamed RIAHI,
As per the Microsoft Document
Initiating a scale up, or scale down action, in any of the flavors mentioned above, restarts the database engine process, and moves it to a different virtual machine if needed. Once the target database engine is ready to process queries, open connections to the current database engine will be terminated, and uncommitted transactions will be rolled back and if you have long running queries, the scaling operation likely to take time.
Thank You!