I don't know that I would call it new, but since it has a new name and I hear they over hauled the menu from when it was San Angel Inn...then I guess new is in order. 
We'll have to wait and see if it's actually any different.
The one from before was not good IMO, living in Texas it will probably take a bit for me to be impressed. Rest assured it wasn't considered authentic or even mediocre at best.
I'm anxious to start hearing reviews on the "new" place.
There is a new Tequila bar there now, they serve appies but not a full menu per say and there aren't reservations taken there. This is called La Cava del Tequila, I hear wonderful reviews.
It is brand new. It is where the outside counter service was. It will not be tex-mex food ( I am looking for where I read this).
I found it on wdwnewstoday:
"Any favorite menu items?
We have three or four spectacular appetizer items at La Hacienda. The trend in Mexican food is appetizers, such as mini tacos. [One that sounds especially delicious is chiles toreados y chorizitos, roasted sweet peppers with lime and sea salt and mini chorizos (pork sausage).] And I really like the skillet entrées skillets with cuts of meat, veggies, seafood, to make your own taco. Im still tweaking the menu!
How does La Cantina menu change?
The Cantina menu will focus on tacos with meat or chicken, properly grilled, and garnished with fresh pico de gallo. We want to create an authentic Mexican taco, something that locals would buy at the market.
Less Tex-Mex, more regional authentic Mexico."
From the MouseForLess site:
" The all-new full-service restaurant, La Hacienda de San Angel located in Epcot, will open daily at 4 p.m. beginning Sept. 16. Reservations will become available later in September for October 15 arrivals and later. Next door, La Cantina de San Angel, the quick-service restaurant, will also open on Sept. 16 and will operate from 11 a.m. to 9 p.m."
"Cantina de San Angel is closed and will reopen as La Hacienda San Angel on September 15. Reservations are being excepted for dinner starting October 15. "