FWIW, you’re under no obligation to convert all of your existing legacy-format Template Sensors to modern format. Legacy format wasn’t deprecated and continues to be supported but it won’t be receiving enhancements. I suggest you consider using modern format when creating a new Template Sensor or when modifying an existing one that’s in legacy-format.
Be advised that legacy format allowed you to define a friendly_name and, optionally, an entity_id (with different values).
In contrast, modern format only allows you to define name which is equivalent to the old friendly_name and it automatically generates the entity_id from that. You can’t define a different entity_id in the entity’s configuration. If you don’t like the generated entity_id, you can modify it afterwards via the UI (editing the entity’s properties).
Because of this difference, converting from legacy to modern can be tricky because if not done right you can lose the entity’s history (because during the conversion, the entity’s original entity_id might not be preserved). For instructions on how to perform a lossless conversion, I recommend this post: