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       y/source/dest/
              Transliterate the characters in the  pattern  space
              which appear in source to the corresponding charac­
              ter in dest.

So you might want to do "sed 
y/ABCDEFGHIJKLMNOPQRSTUVWXYZ/abcdefghijklmnopqrstuvwxyz/" to switch to lower 
and just swap source and dest to switch to upper.

sed rules.

-Peter