Some states will not allow discrimination of pre-existing conditions. Check your state insurance department. Also, getting an MRI does not mean that there will be a diagnosis. You might talk to the doctor to delay the diagnosis until you have switched policies. Diagnosis is usually time consuming and may not be final until the future.
Depending on the diagnosis and any disability that you may have, if you need to take disability, there is a 2 year wait for Medicare (it is a nightmare!). But your student insurance policy maybe extendable. Read the fine print- you may need to continue to take classes, but it may be cheaper...