If you've already utilized batch tracking instead of serial number tracking, it isn't possible to directly switch between the two different types. It's also not possible to use both tracking types simultaneously for the same finished product.
As a workaround, you can switch the type via two different ways:
Create a new item and choose serial tracking from the beginning.
Disable batch tracking for the existing item by choosing No tracking and then enable Serial number tracking.
When turning off batch tracking for an item, all batch tracking information for all variants of that item will be deleted. The same applies to serial number tracking if switched to No tracking.