Break trunk LACP without losing attached DHCP reservation

You could export the config via Import/Export, giving a XML file. The file can include your Interfaces, VLANs and also DHCP server.

Then you adjust your config. 
Then you edit your XML file and re upload it to the firewall.

See: community.sophos.com/.../sophos-firewall-interface-vlan-migration-via-xml-import-export

Unfortunately it doesn’t seem to work. With this you have to unbound the ports first and doing so results in losing all DHCP configurations

Yes, but the config would be stored in your import/export XML file. So you would upload it again. 

Ah. That link was talking about exporting interfaces only ? I didn’t see dhcp in there 

Yes - it was just an suggestion, how to approach that. XML Import/export supports everykind of approach. 

Ok I’ll look into the options available. Trying to export everything seemed to just crash unfortunately 

How can an EXPORT crash something on your firewall?

You sure mean your subsequent import after editing?

The export itself crash. As to how I wouldn’t know. 

I never had a "crash" when exporting from XG/XGS systems. What do you mean exactly?

The export itself doesn’t go to the end. It never finishes. The process does not finish. 

not sure I understand your question tbh.