I just wanted to share my experience which I think can be helpful.
Around 16 months I had to move my DD to one nap. I felt that there was just not enough day time to give her 2 naps without constantly waking her up. Intuitively I knew, She was waking up underslept all the time, especially from her 1 st nap (one caveat, she never would go to sleep easily next time if she was not tired enough, so taking that 2nd nap was becoming a struggle for us too, her 1 st nap was easy though)
It was stressful to do that in just one shot from 4 hrs A to 6 hrs A, because I didn't want to rush before 18 months and I was always panicked about OT. But then...we tried to keep her up for 6 hours (slightly less if she didn't wake up in the morning happy) and put her to bed for nap and it got extended to 2 hours in just a few days.... We did some early night bedtime when she looked too tired but it was very quick. We never looked back at 2 naps. Just made little tweaks around 1 nap schedule.
It was so easy, I could not believe, the easiest transition for us. She was rested during day and night.