As it stands, it is impossible to rename a given task written with C++.
The name is correctly serialized, but hidden with no option to change it.
/**
* Base struct of StateTree Conditions, Evaluators, and Tasks.
*/
USTRUCT()
struct STATETREEMODULE_API FStateTreeNodeBase
{
...
/** Name of the node. */
UPROPERTY(EditDefaultsOnly, Category = "", meta=(EditCondition = "false", EditConditionHides))
FName Name;
...
}