There were a few of the -436 converted to all-economy
I don't think it was a big deal to reconfigure an aircraft. Around 1969/70 the all-Y 707s (162Y) did the daily LON-Prestwick-Toronto and also occasional extra sections on the Toronto nonstop route in peak Summer. The standard configuration was 16F/130Y but other combinations were used occasionally, anywhere from 8F to 24F (in multiples of 4, obviously).
There were also all-Y configurations for the STVs and SUVs as well (from memory 117Y and 157Y) but they were very rarely used.