Your chances absolutely change once you get an interview! (so good job!) As far as how much it changes, that really depends on a few things. I've heard of programs doing it differently, so I can't speak to those... but in my program about 25ish people typically interview for 4-6 spots. The important factor in that is the # of people applying for your particular POI. Last year we had say 3-4 per POI, except one POI interviewed like 8 (!) So you see, the chances are really different for those applying under that POI. When I interviewed, I was competing against 3 others - which means I had a 25% chance. Then it turns out that my POI secured extra funding and had 2 spots! I was actually the 3rd choice, but since the 1st choice chose a different school, I was admitted along with another person.
Also, some programs do not feel that they have to pick someone. I think for some places, they'd rather not fill the spot than admit someone who was clearly not a good fit for the program. [for instance if their first 1-3 choices turn it down, they may not go all the way down the list] This happens much less often, but it does happen. This makes your odds especially difficult to figure out.
