@S-K 

This would be an expected behaviour, although not entirely desirable..   I have seen other vendors cause the same during NAT changes. 

Basically, when you are updating any NAT rules it has to recompile the sessions to ensure that the new rules apply to existing user sessions, the only way to do this is the terminate the sessions active.. which will cut the users off their connections.

NAT rules are designed to be applied out of hours or during arranged downtimes, this would be the best practice. 

@Philbert well I disagree with this. There are also solutions where changing nat rules won't reset all connections. Even Sophos UTM which is old as the hills can handle this. And even if there were technically no other possibility, there should be at least a hint before a configuration change would usher all connections into the great digital beyond.

@S-K 

Yeah agree its not the best, but sadly the way it is... 

For the record, I only ever experienced this in hardware based NAT rules, software rules (like Sophos you mentioned) don't appear to display this issue.