START
...
...
END
START
...
...
END

sed '/START/,/END/{/START/{h;d};H;/END/{g;s/.../foo/gp};d}' file
START
foo
foo
END
START
foo
foo
END

awk '/START/{p=1;n++};p{if(n==1)sub(/.../,"foo");if(n==2)sub(/.../,"bar");print};/END/{p=0}' file
START
foo
foo
END
START
bar
bar
END