Hi Anil,
2nd approach will be better, becz DR systems are usually installed on remote locations so that in case of any disaster we can use it to recover. But if your customer is insisting on the 1st approach then to change instance number of QAS better way is to do homogeneous system copy and changing the instance number.
If you are going to change the instance number manually,
For ABAP stack you need to
1. Rename directory in /usr/sap/<SID>/SYS/profile/DVEBMGS<old_inst_no> with DVEBMGS<new_inst_no>
2. Change the instance number in START, INSTANCE and DEFAULT profile. (make sure before changing take backup of these files)
3. You need to make changes in /etc/services file for sapgw<inst_number>
4. Look for files containing number 00 with following command in /usr/sap/<SID>/ and /sapmnt/
# Grep 00 .[a-z]* If you find any file containing the old instance number change it to new instance number.
4. Take restart of the system.
For JAVA part as I suggest to do homogeneous system copy with new instance number.
Regards,
Vivek Sonnekar