It is not necessary to apply for a design patent again for product upgrade. In the case that there is a small gap between the appearance of the new product and the original patent, it is still within the protection scope of the original patent. Due to lack of innovation, submitting new applications may not be authorized. Even if authorized by luck, the rights protection stage may be deemed invalid by others because there is no obvious difference from the original patent.
If only a certain part of the appearance has changed during the product upgrade, you can apply for a design patent for this part alone. As a separate component, its shape change is enough to distinguish it from the existing design, and it is easy to get authorization.