Uncontrolled depression

Patient: My wife who is 70 years old suffers from dementia, my main concern is with her depression, she has been on Lexapro 20 Mgs. for a year, however I don’t see any improvement after all that time on the drug, recently y told her general practitioner that I wanted her to swith to a different drug I suggested Prozac, and he agreed and gave me a prescription for 20 Mgs. pills, my question is do I need to wean her from the Lexapro before I start giving her the Prozac? if so how should I go trough the tapering off schedule? I also give her Xanax (low dose .25 Mgs. three times a day) when she doesn’t stop crying, usually in the afternoons.Please advise