ChefSid -- "im in the preview process of switching to another post office out of state. when and how should i tell my P.O. and pm?"
-- "Morty" has given you a couple of options.
-- Once your new post office accepts your transfer request, you really don't have to do anything as your new office just has to notify your current post office. Step 4 ( P-115 ).
-- However - comma - you never know, it might be necessary to come back to your current office ( never know what the future holds as Karma can be a, well you know! As well as that "burning bridges" axiom ). So giving your current office a heads up could be considered the polite and proper ( and professional ) thing to do.
-- Once your current office has be notified by your new office, the transfer will be effective within 30 days. This is where the polite and proper thing comes in to play. Your current office could keep you for the full 30 days and no one can do anything about it. If a heads up is given, you could be on your way earlier, much earlier.