You are thinking of switching to Ritalin. What does your doctor say about the change? Does he recommend it?
You will certainly get a positive test for stimulants. However, from what I understand, if you have a perscription - it should not be a problem due to the Americans with Disabilities Act of 1990. But, I would certainly contact your HR department and find out their policy.
A decent link here if you have not already seen it - http://prescription-drug.addictionblog.org/does-ritalin-show-up-on-a-drug-test/
Hope this helps.