Ok. Thanks again. I think I've found what I need. (on lines of 'inpch' that contain 'TH' I change the field at position $pos to be $new, preserving spacing made of 'space's and/or tabs). If anyone comes up with sth. simpler...
Ruda (The script is for 'rc', btw., so that the concatanation on the 3rd line work.) 9 awk ' /TH/ {n = split($0, spaces, /[^ ]+/); $'$pos'='$new'; line = ""; for(i = 1; i < n; i++) line = line spaces[i] $(i); line = line spaces[n]; print line; next;} {print} ' inpch