What's the significance of head/neck ultrasound finding showing enlarge cervical lymph node (17x7mm) has changed from hyper to hypoechoic?

Non-specific: An enlarged lymph node in the neck could be secondary to anything from a harmless infection to to a cancer. You will want to be carefully evaluated to ensure that this is not anything serious.

Possibly reactive: Ultrasound indicates size and location of masses, rarely the actual cause. Hypoechoic node may indicate reaction to infection but frequently a ct of the neck with IV contrast is needed to search for cause of the altered anatomy of the lymph node. Usually a recent sore throat or dental problem is the source. Ask your doctor if a neck ct is indicated.