Do I modify existing or delete/add External (WAN) Interface with new Internet provider?

i mostly reconfigure existing interface.

Only if i have to use booth providers for some time (while migrating ISP) i create a new IF and delete the old afterwards.

As long as you do not absolutely want the boxes interfaces named right (i mean the description under the ports of the hardware box) e.g. LAN/WAN i would use a new HW interface and leave the old one untouched till the new connection works fine, then delete the old one.

By disabling/enabling the "default gateway" checkbox you can easily shift between the both lines. I would not recommend enabling two gateways/uplink balancing/multipath in a temporary behaviour.

If the names are important for you you can - when the old connection is no longer needed - delete this one, change the HW interface of the new connection to eth1 and un-/replug your external cable in the right port.

If you edit an existing interface you have to care about the advanced settings, too if your new connection needs e.g. other MTU.

As long as you do not absolutely want the boxes interfaces named right (i mean the description under the ports of the hardware box) e.g. LAN/WAN i would use a new HW interface and leave the old one untouched till the new connection works fine, then delete the old one.

By disabling/enabling the "default gateway" checkbox you can easily shift between the both lines. I would not recommend enabling two gateways/uplink balancing/multipath in a temporary behaviour.

If the names are important for you you can - when the old connection is no longer needed - delete this one, change the HW interface of the new connection to eth1 and un-/replug your external cable in the right port.

If you edit an existing interface you have to care about the advanced settings, too if your new connection needs e.g. other MTU.

Thank you! I think this time I will just modify existing interface if that's all I need to do. I thought that possibly creating a new interface each time was maybe a best practice... that it would update the software properly and I won't have to worry about bugs later from taking a short-cut. :)

Hi, Sean, and welcome to the UTM Community!

Sachin's prescription is the best practice.  Off to the right of your interface definition, you will see a blue button - proess on that and you will see all of the things you would have to massage if you were to create a new interface definition.

Cheers - Bob