I think the above poster might be right that someone might have been recommending Desert Inn (which is on the Harbor side and right across from the Disney esplanade) because YES, Discovery Inn & Suites is next door to BW Stovall's, which is on the Katella end and a farther walk. If you decide to keep your original reservation with BW Stovall's, I can just tell you we've had two great experiences and we LOVE it in spite of the little bit farther walk. I had once contemplated staying at Discovery Inn, but after reading some pretty alarming reviews we chose BW Stovall's and I'm glad we did, I'd have no hesitation recommending them. (You can still find poor reviews on Trip Advisor about Stovall's also (heck you can about the Grand Californian as well) but they are more few and far between, they are not what most people seem to experience--if you want a nice, clean, affordable, place to rest your head with a beautiful pool area (that has never been crowded on our trips), with complimentary breakfast and wonderful customer service most people would be happy here.
As one of the above posters mentioned if you enter at the DTD entrance near
Disneyland Hotel (which never has a big lineup) the monorail entrance is right there and that is very convenient and once you scan through you are already in the park. You don't have to go through security and then wait in line again at the gates to enter the park as you do with the Harbor entrance. We used the monorail several times on our last trip and really liked that. You also can still cut through the Grand when returning to Stovall's at anytime, although it looks like you're just staying a day/night so you probably won't be doing much coming and going in that short timeframe.
Enjoy your trip!