You can't without a player wipe. To be fair it's possible to do but would mean editing all the player files you have, which could be considerable depending on the size of the server.
Making holes open in original FL that are normally shut, needs to be decided when you set the server up initially. As to how to do it, this has been covered a million times, but look in your initialworld.ini file and place a colon ; in front of the gate you wish open.