This is an example of the "replace pattern" command.
You need to change of an existing VPN
[edit]
root@KB# show configuration security ike | display set
set security ike gateway GW-VPN-TEST1
root@KB# show configuration security ipsec | display set
set security ike ipsec VPN-TEST1
root@KB# replace pattern TEST1 with JUNIPER
set security ike gateway GW-VPN-JUNIPER
set security ike ipsec VPN-Juniper
root@KB#commit
Note: This will flap the VPN.