cat input.asciidoc | awk -f replace.awk > output.asciidoc

input.asciidoc

Header

Subheader

SubSubHeader

blabla

SubSubSubHeader

Next SubSubSub has an error (on purpose).

SubSubSubHeader2

Subway

Done.

output.asciidoc

Header

Subheader

SubSubHeader

blabla

SubSubSubHeader

Next SubSubSub has an error (on purpose).

SubSubSubHeader2

Subway

Done.

replace.awk

BEGIN {
  last = -1
  chars[1] = "="
  chars[2] = "-"
  chars[3] = "~"
  chars[4] = "^"
  chars[5] = "+"
}

{ if (last != -1) {
    l = length($0)
    if (l > 0 && l == length(last)) {
      pl = 1
      for (i in chars) {
        h = sprintf("%" l "s", "") ; gsub(/ /, chars[i], h)
        if ($0 == h) {
          prefix = sprintf("%" i "s", "") ; gsub(/ /, "=", prefix)
          last = prefix " " last
          pl = 0
          break
        }
      }
      if (pl) print last
    } else {
      print last
      last = $0
    }
  } else last = $0
}

END {
  print last
}