OK, I think there will be no problem. The new SMS gateway will be the SG VIP interface, say 192.168.2.100, so for the SMS to reach the SG IPs in segment 192.168.3.0/24 I only need to configure rules between these two segments and open those ports, and after that migrate the SMS, and then restart SIC.
A collegue told me SIC must be established between the SMS and SG "nearest" interfaces, which mean here the SIC "should be" established between the SMS (IP in segment 192.168.2.0/24), and SG IPs in segment 192.168.2.0/24. But considering all we talked about, SIC can be established between the SMS (IP in segment 192.168.2.0/24), and SG IPs in segment 192.168.3.0/24.
Regards,
Julián