Welcome to the A/P forum (if you haven't been before).
I'm not a professional, but in my experience you do need to come off one anti-dep medication entirely before starting another. Usually this is done by tapering down the dose slowly and gradually under supervision. Sometimes you do get some withdrawl symptoms, but if you do it slowly they are minor.
Once one med is out of your system then you can start another. Now I may be wrong here, but this is my experience of switching meds in the past. I hope your husband finds that Lexapro is a better med for him. Best wishes,
Panic Attack Survivor
Weekend Warrior Princess