Search for a string in a line and replace it with another string from another file - Shell scripting

Hi Team,

I am writing a shell script which will procedure three output files

File: Source_Lun.txt
ud1001
ud1002
ud1003
un1001
un1002

File: Dest_Lun.txt
ud2001
ud2002
ud2003
un2001
un2002

File: CloneLuns.txt

/vol/clone_vol_rtb_bed_serv1_data_03_12_2014_00_19/qa_ud1001/rtb-bed-serv1_ud1001
/vol/clone_vol_rtb_bed_serv1_data_03_12_2014_00_19/qa_ud1002/rtb-bed-serv1_ud1002
/vol/clone_vol_rtb_bed_serv1_data_03_12_2014_00_19/qa_ud1003/rtb-bed-serv1_ud1003
/vol/clone_vol_rtb_bed_serv1_data_03_12_2014_00_19/qa_un1001/rtb-bed-serv1_un1001
/vol/clone_vol_rtb_bed_serv1_data_03_12_2014_00_19/qa_un1002/rtb-bed-serv1_un1002

Now, I want to produce the forth file which should replace the last string from file CloneLuns.txt with a string value from Dest_Lun.txt

for example:

Take a first line from the file CloneLuns.txt,
"/vol/clone_vol_rtb_bed_serv1_data_03_12_2014_00_19/qa_ud1001/rtb-bed-serv1_ud1001"

Take a first line from file Dest_Lun.txt,
ud2001

I want to replace only string ( ud1001) with ud2001 from both files and write it to another file ex: modified_luns.txt

so the output should be,

"/vol/clone_vol_rtb_bed_serv1_data_03_12_2014_00_19/qa_ud1001/rtb-bed-serv1_ud2001"

and second line
and third line
and forth line so on ...

Can someone shed some light on it?

Thanks
Shankar