user@bkphost:/tmp$ cat test.txt
As123312123 asdasdadasqsef3w4rfweweweffwerfew
As123312123 asdasdadasqsef3w4rfweweweffwerfew
i12311231231231 HOSTA
As123312123 asdasdadasqsef3w4rfweweweffwerfew

user@bkphost:/tmp$ echo $h
HOSTA
user@bkphost:/tmp$ echo $k
ssh-rsa AAAAB3NzaC1yc2 root@HOSTA

user@bkphost:/tmp$ sed -i "s#.*$h#$k#gI" test.txt

user@bkphost:/tmp$ cat test.txt
As123312123 asdasdadasqsef3w4rfweweweffwerfew
As123312123 asdasdadasqsef3w4rfweweweffwerfew
ssh-rsa AAAAB3NzaC1yc2 root@HOSTA
As123312123 asdasdadasqsef3w4rfweweweffwerfew