Acoustic neuroma definitely does not convert to brainstem glioma. Because the resolution (image clarity) of CT scan is not as good as MRI, it may look like a brain stem glioma.
MRI and CT are suggestive of a problem, and final diagnosis is based on the tissue examination (Histopathology) that is sent after removing it surgically.
Next Steps
You need to consult a Neurosurgeon and undergo surgery.
Health Tips
Don't delay, your scan mentions hydrocephalus, which means raised pressure.