When executing ‘search-and-replace’ commands on ASCII under Linux, Unix (or *nix) operating systems, sed works or most cases and makes for reasonably straightforward reading of the script.
If you want to replace multiple lines of text with multiple lines of text, following is a perl one-liner that does the trick and is much easier to wrangle than trying to do it in sed.
perl -i -pe "BEGIN{undef $/;} s:${EXISTING_LINES}:${REPLACEMENT_TEXT}:smg" file.txt