Hello, i am bit stuck with making script for automatic procedure.

Case:

Two files. One is flat file, other is csv file.
csv file has two column/fields with comma delimited data.

Here is what i need (explained way)

CSV file:

field1 | field2
"hello","byebye"
"hello2","byebye2"
"hello3","byebye3"

Flat file has matching data:

"hello"
"hello2"
"hello3"

If found:
"hello"
"hello2"
"hello3"
..in flat file, then it must be replaced from csv file with FIELD2 according
to search match.

But i have to search "hello" from csv file field1 data with flat file together and if found match in
flat file, it must be replaced from csv file with field2 data.

I know i could split csv file to two files, one containing field1 data and
second file field2 data but that would be not so dynamic way.
Though then i could easily use loop with cat + sed

Idea is to make nagios mass updating hosts naming changes in config.
So im wondering could i do it with awk or sed more universal way?

Any examples would be great help.

Thanks in advance..

Frankie