How to switch Lexapro to Prozac?

I just saw my Dr. Today because I am switching from Lexapro to Prozac because of weight gain on Lexapro. This is coming from my Dr. You MUST NOT MIX LEXAPRO and PROZAC. They are not compatible chemically. You must allow at least 3 or more days for the one drug to "wash out" of your system before you start the new one. You may need more time than that. Confirm this with your Dr.