You can actually upgrade to PS4 anytime, even if you don't have a PS4 yet (see http://na.finalfantasyxiv.com/pr/ps4/ps3_users/). You'll just lose access to the PS3 version immediately. If you're cutting it close with your PS4 acquisition and don't mind being unable to access the game for a while, you can log into Mogstation to upgrade to PS4 anytime before Dec 31 2014. You'll get a code to download the PS4 version as soon as you have a PS4 to input it on.

As for Heavensward, that will be a brand new retail disc (and digital download) that will only function if you already have the base game. When it comes out in March/April/May, you should buy it on whatever console you're currently playing FFXIV on.