Hi Jimmy,
On September 6, 2023, Any classic VM that has not been migrated to ARM will be stopped and deallocated. This means that you can't make a backup before migrating.
Migrating to ARM will not have impact on your data, in order words the move will happen smoothly without data loss.
For more info -> https://learn.microsoft.com/en-gb/azure/virtual-machines/classic-vm-deprecation#what-resources-are-available-for-this-migration
Please let me know if you have any other questions!