Thank you @nielsr for being responsive.
That error is resolved now, but the question is "does simply changing the number of labels mean that we have changed the classifier head?!"
By the way, for my problem, I had to do these modifications:
model_name = "dslim/bert-base-NER"
mymodel = AutoModelForTokenClassification.from_pretrained(model_name, num_labels=len(my_id2label), ignore_mismatched_sizes=True)
...
mymodel.config.id2label = my_id2label
mymodel.config.label2id = my_label2id
mymodel.config._num_labels = len(my_id2label) ## replacing 9 by 13
mymodel.config.num_labels = len(my_id2label)