I tried the following code. I cannot figure out why the page size remains the same. As such, the papersize remains A4 portrait instead of switching to landscape.
\definepapersize[standing][A4] [A4]
\definepapersize[lying] [A4,landscape][A4,landscape]
\showframe
\starttext
\setuppapersize[standing]
Page 1: standing pages from here on. \page
Page 2. \page
\setuppapersize[lying]
Page 3: lying pages from here on. \page
Page 4. \page
\adaptpapersize[standing]
Page 5: adapted to standing \page
Page 6: automatically back to lying \page
\stoptext
