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The 'IP Selection by Remote Peer' Link Selection configuration is only used for other gateways managed in the same SMS. So as you say, when you update the main IP and install policies to all your locally managed gateways, they will use the new IP. Third party externally managed gateways have the external IP of your side manually configured on their end. So as long as your old IP is still being routed to your new IP, they should still connect. You may have to take note of any hideNATing in the communities though as if you're doing any 'hide behind gateway' stuff, that will change when you change the gateway IP.

Now, your outbound traffic to the VPN peers may be an issue here, as you will be changing your gateway's source IP. You may be able to solve this with manual NATs, but you may also have to change your outbound NAT configuration as by default a gateway won't NAT its own outbound connections. See: https://support.checkpoint.com/results/sk/sk63805

To get around this, if it's possible to set your old IP on a new interface and then set static routes out this interface to each of the VPN peers, that would be the way to go. No manky NAT problems there. The default outbound link selection uses the OS routing table, so it will use the IP of the interface the VPN packets egressed from. Hence your old IP will be used for anything routed out that old IP interface.