there's a server A and server B which are configured as HA and their gateway

Is your FQDN going to remain the same or is the new server going to have a different FQDN. Remember that your topology file will hold this info (or the IP address based on how you've configured).
My personal recommendation is to shut-down both the engines for such a config change.

Thanks for recommend. But I must keep one server up. Shutting down is the last option if no option left

The preferred approach would be to follow what Sathya recommended in the last note.

If you MUST keep one server up, it is possible to do so. But you will need to eventually schedule a downtime before you bring the engines up on Server B, since the engine database files needs to be synced.

Here’s what you can do:
1)          Check-point the environment when both servers are up
2)          Stop SERVER B
3)          At this point all the gateways on SERVER A will switch to [STANDALONE] to handle all the requests/transactions
4)          Make any necessary server/configurations changes that you like for SERVER B
5)          Once you are done with the changes, use “stop-suite.bat|sh” to stop the engines on SERVER A.
6)          Stop-suite.bat|sh will ensure that the kdb is copied over to both /gw1 and /gw2 directories and both gateways will be on the same engine database id (no mismatch in kdb's).
7)          Ensure that that file was correctly copied over to the NFS that is hosting the kdb’s by looking at the /gw1 and /gw2 directories.
8)          If the result of the above step is satisfactory, start engines on SERVER A
9)          Start engines on SERVER B

This will significantly reduce the downtime in your environment, while you make the SERVER change.

Hope this helps!

Thanks for recommend. It gave me a idea.
What if I want to change server A too? After changing server B!

Do I stop the service and jboss on server A after checkpoint and redo the steps on your walk through once again on server A? But I doubting.

After step #7 from my previous note, you can follow these steps:

8) Ensure the kdb's were copied over correctly, after the stop-suite.bat|sh so that both /gw1 and /gw2 directories have the same kdb.
9) Start engines on SERVER B and JBoss so that SERVER B can work in STANDALONE while you perform maintenance (server+configuration changes) on server A.
9) Once that is complete, stop Jboss and run a stop-suite.bat|sh on SERVER B, so that kdb's are synced between SERVER A and SERVER B
10) Verify that the kdb was copied over to both /gw1 and /gw2 directories
11) Start SERVER A and wait for all engines to come up
12) Start SERVER B and wait for all engines to come up

Thanks a million... great help