Existing connections that are NATted will keep using the same NAT, even if policy is reinstalled with rules specifying a different NAT address/rule for that connection. The NAT address to use is determined right after the Firewall/Network policy accept of the first packet and cannot be changed for the life of the connection. Only newly-initiated connections will use the newer rule.
What is probably happening is your DNS servers are sending DNS UDP requests to an ISP forwarder, which is tracked as a "connection" by the firewall. Even if you change the NAT rule and reinstall policy all those packets will still have the old NAT applied until the DNS "connection" ends and a new one starts. This would be a use case for clearing the NAT table as described earlier in the thread.
A better way to do this without deleting more than necessary is to add a new SAM rule matching the connection attributes in the SmartView Monitor (or fw sam) and making sure "close connections" is set. Simply apply the SAM rule, then immediately remove it to force new connections (with the new NAT) to start.
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