They don't switch foods. Basically, there is breakfast (which has some brunch-type options in the enormous buffet, such as cold cuts, chicken, pizza, hot dogs, etc.) and there is dinner. But there is no separate lunch meal - if they started rolling out a separate set of food items for lunch, then they would charge differently.
I even checked with a CM in
Disneyland Dining about this very topic earlier this year - I asked him specifically if there was any different food put out for brunch/lunch and he said no. Now depending on the season, they may have different items in the buffet - there could be slightly different things during Summer than what's there during Fall or Winter. Also, there might be a couple of different buffet items on weekends that are not there on weekdays, but, essentially, what you see at 7:00 a.m. is pretty much what will be there at 11:00 a.m. or noon (except they will put out fresh batches of it, of course!!).
So right now, the "breakfast" extends from 7 a.m. to noon on some days, until 1 p.m. on other days, and until 2 p.m. on certain other days (like on Saturdays or during holidays). Is your Sunday a day when they serve until noon or later than that? You want to make sure you get enough character time - sometimes the characters will start trickling out of the restaurant a bit before they stop serving.
During off-peak season (which would usually be now), the breakfast/brunch price for adults (including tax and tip) is about $32 and some change . During peak season (like during the holidays), the breakfast/brunch is about $37 and some change for adults (including tax and tip).
In the old, old, old days of Goofy's there actually was a whole separate lunch (spacemermaid probably remembers this). they had a breakfast meal, a lunch meal and a dinner - all for different prices. Of course, back then the breakfast was also only $11.00 so you can tell how long ago that was!!!

For some reason or other, when they moved to the different location in the DLH and when DLR began adding in more character meals, they stopped doing the lunch at Goofy's.