UNIX for Advanced & Expert Users

I have a requirement:
I got a perfect example in this forum and looks like it would be quite similar but I have an additional task to do. I would explain.
I have a file x.csv
client_id,client_nbr
ABC,1250
CDE,1520
EFG,1000
PQR,1800
I have a file y.csv
client_id,client_nbr
ABC,1000
PQR,1500
XYZ,4500
I did a lookup based on the nice script below [I got in this forum] and got an output as below:
awk -F, -v OFS=, '
# load array with changed client numbers
NR==FNR { client_nbr[$1]=$2; next }
# if this client has changed, print updated record
$1 in client_nbr { print $1,client_nbr[$1]; next }
# otherwise just print the current record
1
' x.csv y.csv > newfile.csv

My newfile.csv output is
client_id,client_nbr
ABC,1000
CDE,1520
EFG,1000
PQR,1500

Now my requirement comes:
if you see in y.csv I have a row XYZ,4500
I need to insert it also alongwith the lookup update we did with the above script. Can u plz help me? I am stuck.