Why shell to sed at all?

while ( <> ) {
    s/priority /priority\n/;
    print;
}
[download]

If you want it all in one line:

perl -i~ -pe 's/priority /priority\n/'
[download]

That second when given cos2 as an argument will read the input from cos2, write the output into cos2, and save the original contents as cos2~.

You might want to see s/PATTERN/REPLACEMENT/msixpogce, perlre, perlrun (for the one-liner options), and perltrap which covers traps for people coming from awk, sed, C, and other languages.