Hi Sarah,
My DD went down to 1 nap at about 14 1/2 mos. or so and we struggled with the same issues (and still are to some extent!). She started refusing her morning nap, so after trying unsuccessfully to keep it in place for a couple weeks, I ended up just going cold turkey and keeping her up till 12:00 or so, giving her lunch beforehand. For the first 1 1/2 weeks or so, she would take her nap but only sleep for 40 min.
. We put her to bed a little earlier on those days, but not super early. After that week and a half, things were GREAT for about 2 weeks and she would take 2 or 2.5 hour naps in the afternoon and sleep 11 hours at night. That was a dream! Things got thrown off by having company in the house and we started an overtired cycle which we're still dealing with now, but in our case it really didn't take long for her to adapt to the 1 nap per day. If your dd has previously taken 2.5 or 3 hour naps starting at 12, then you probably won't have the 40 min. nap issues that we originally had. In our case, the amount of awake time our dd needed and could handle after switching to 1 nap increased really quickly. Less than 2 months ago, she could wake from a 2 hour nap at 4:30 and be ready to sleep again at 7:45. Now, she really needs to be awake at least 5 hours or so to be able to fall asleep easily. Not sure if this is the case with everyone, but that's what happened with our dd. I also took one of the suggestions I found on these boards and did a quiet reading time with our dd during her previous morning nap time (she had only been sleeping 30-40 min. in the mornings before refusing the nap completely). This seemed to help carry her through till lunch. Good luck and hope this was sort of helpful