If the phrase split across a line break does not count then it is easy. If it does count then it is rather hard to do in a one liner without being clunky..

# No worries about line breaks
perl -i -pe's/Will Emigh/YAPH Emigh/g' 

# line breaks concern me but STDOUT will do
perl -pe'$a.=$_}{$_=$a;s/Will(\n*) (\n*)Emigh/YAPH\1 \2Emigh/g'

# line break concern me and lets put it back in the file
perl -ne'BEGIN{$f=$ARGV[0]}$a.=$_}{$_=$a;s/Will(\n*) (\n*)Emigh/YAPH\1
+ \2Emigh/g;open F,">$f";print F $_'
[download]

Of course the last only works with one input file, and I got so worried about line breaks I forgot to make it short. (cunning tachyon)

Cheers,
R.