As for when & where, I'll leave you to Google-search that one, but the details of what's changed are actually really cool!
No rules changes, only additions
The biggest thing to know about the new Core set is that there are no rules changes, no new templates or dice. There are a couple of new maneuvers for the ships like the new Tallon Roll, but they're just like the additional actions introduced in expansions like the SLAM or Cloak rulings: just adding more flavor and options for ship and squadron design.
This is not your father's TIE fighter or X-Wing
"your father"... see what I did there??
The TIE Fighter included in the original Core set is the TIE/ln starfighter (the original TIE from Episode IV), but the TIE Fighter included in the new Core set is the TIE/fo starfighter, the version from the new Episode VII movie coming in December. Likewise, the X-Wing in the original Core set is the T-65 starfighter (commonly called an X-Wing due to the "X" shape the S-Foils make), while the one included in this new Core set is the T-70 starfighter also from the new movie. The stat lines, point costs, upgrades, pilot skills, and pretty much everything else are slightly tweaked to make these new ships distinct and different from their Original Trilogy counterparts without invalidating the originals in game play.
If I own the original Core set, should I buy the new one?
In my opinion, it's worth having two core sets just for the extra dice and templates, and this is a great way to get those and some of the new ships as well. If you already have two of the original Core sets, it's up to you as to whether it's worth buying this Core set, or just buying the individual ships when Wave VIII comes out.
