Can benign thyroid nodule become malignant?

Yes: Rarely. But possible. Need to have them evaluated and removed, if there is a suspicion. By putting a thin needle in the nodule and taking fluid for examination (fna) usually helps in making the decision whether it needs to be surgically removed or just followed.

Well...: A thyroid nodule is technically not "benign" unless it's surgically removed and the pathologist found no cancer in the specimen. A nodule that is still in your neck could still be cancerous despite a "benign" biopsy because not all of the nodule is cancerous. Sometime it's just a speck that is cancerous and was missed during the biopsy. That's why it's important to continue to follow the nodule.