The 'What' and 'When' can be traced via admin log on the master server, but since all changes are done by an Administrator/root, the user name will not be logged.
I have just done a test on my test system - changed 'keep logs' from the default 28 days to 20:
bpconfig -kl 20
admin log:
22:36:53.943 [18851] <2> bpconfig: INITIATING: version NetBackup 6.5 created: 2009050106
22:36:53.943 [18851] <2> logparams: -kl 20
...
22:36:53.991 [18851] <2> vnet_vnetd_service_socket: vnet_vnetd.c.2060: service: bpdbm
22:36:53.996 [18851] <2> logconnections: BPDBM CONNECT FROM 192.168.120.4.51350 TO 192.168.120.4.13724
22:36:54.330 [18851] <2> bpconfig: EXIT status = 0