fluoxetine and lexapro are both SSRI type so an overnight swap is usual however I doubt it will work either and would be thinking of trying another anti depressant of a different type, such as effexor, whgich is a SNRI type
there are lots of differerent types of antidepressants and several types may need to be tried to find 1 that works for a person, sadly many docs dont seem to realise this, its not unusual to hear of someone who has been tried on 4 or more antidepressants all of the same type and still with no help from any
another common mistake is for doc to forget to raise the low starting dose later 
some meds are promoted as 'add ons' to get an antidepressant working but I dont think they help
recovered former longtime anxiety and panic attack sufferer and helper of other sufferers but no training or qualifications in medicine or psychology, any remarks that may be taken as advice must be confirmed with doctor or other health professional