Shell Programming and Scripting

Now to the addressing. You still haven't said a word about the number of lines per section.
This substitutes in lines 1...1 and in lines 3...3 only (as you have given for header and trailer in your last example)

Code:

sed '
1,1 {s/\(.\{1\}\)../\1yyy/; s/ $//;}
3,3 {s/\(.\{1\}\)../\1yyy/; s/ $//;}
' file

The same is to substitute in all lines but lines 2...2 (the detail section in your last example):

Code:

sed '
2,2 !{s/\(.\{1\}\)../\1yyy/; s/ $//;}
' file

And it deletes a space at the end.
Of course a contents-addressing can solve it, too

Code:

sed '
/^.AR/ {s/\(.\{1\}\)../\1yyy/; s/ $//;}
' file