If you just want to change the active server for CORPORATE_V1001_DHCP_RELAY and VOICE_V1103_DHCP_RELAY, then you need to specify the active-server under the group only as you already configured.
set forwarding-options dhcp-relay group CORPORATE_V1001_DHCP_RELAY active-server-group BLUECAT_SERVER
set forwarding-options dhcp-relay group VOICE_V1103_DHCP_RELAY active-server-group BLUECAT_SERVER
You dont need the below config if you dont want to chnage the server for other groups.
set forwarding-options dhcp-relay active-server-group BLUECAT_SERVER
Remove this and add the original server-gorup "set forwarding-options dhcp-relay active-server-group DHCP_SERVER"
NEW config:
set forwarding-options dhcp-relay server-group DHCP_SERVER 10.172.26.17
set forwarding-options dhcp-relay server-group BLUECAT_SERVER 8.8.8.8
set forwarding-options dhcp-relay active-server-group DHCP_SERVER
set forwarding-options dhcp-relay group CORPORATE_V1001_DHCP_RELAY active-server-group BLUECAT_SERVER
set forwarding-options dhcp-relay group CORPORATE_V1001_DHCP_RELAY interface reth1.1001
set forwarding-options dhcp-relay group CORPORATE_V1002_DHCP_RELAY interface reth1.1002
set forwarding-options dhcp-relay group CORPORATE_V1003_DHCP_RELAY interface reth1.1003
set forwarding-options dhcp-relay group CORPORATE_V1004_DHCP_RELAY interface reth1.1004
set forwarding-options dhcp-relay group VOICE_V1101_DHCP_RELAY interface reth1.1101
set forwarding-options dhcp-relay group VOICE_V1102_DHCP_RELAY interface reth1.1102
set forwarding-options dhcp-relay group VOICE_V1103_DHCP_RELAY active-server-group BLUECAT_SERVER
set forwarding-options dhcp-relay group VOICE_V1103_DHCP_RELAY interface reth1.1103
set forwarding-options dhcp-relay group VOICE_V1104_DHCP_RELAY interface reth1.1104