Did you check the help section of the PvZ 2 game you have? I don't know where it is on the iPad version but in the Android version it is accessed thru the wrench icon in the very first menu that has the game's title on it. On the Android version the help button is a link to a webpage explains various FAQs including how to change your plants costume. I haven't yet tried it out but it says that you can toggle the costumes on and off in the Almanac or the plant selector view for all the plants except the Sunflower, Peashooter and Wall-nut. This at least on the Android version.
Hope this helps.
