Can I substitute my cat's food that was medically prescribed for urinary blockages with the Purina brand?

My cat is currently eating Urinary SO 33. I found Purina One Special Care Urinary Tract Health. From what I can tell they do the same thing. With the prescription food I'm paying nearly $5/lb. With the Purina brand I'd be paying $2/lb. I'm on an intern salary and the Purina brand would be favorable, but my cat also nearly died this past February due to a urinary tract blockage. Any thoughts or suggestions would be appreciated.