I think I posted this is the wrong forum. Could you please move it to the "PI System Integration" Forum, or the forum it belongs.
I think you can read Kenneth's post in this thread, where he shared some of his experience of working with a HA server.
As for your questions, I think whether or not you need to upgrade the interface would really depend on what you have currently. I would say that the main requirement on the interface side after the you move to a HA server is that you need to configure buffer to fan the data to all the PI HA collective members. So you probably will be installing the PI Buffer Subsystem and configuring it appropriately to get it working.
PI Buffer Subsystem will understand that the server is a collective from the PI SDK setting on the interface node, and automatically know it should fan data to all the collective members.
If you need more information, I would recommend that you take a look at the Library section for the documents under the path: vCampus PI Product Kit -> Server Products -> High Availability
Hope this helps
Thanks I'll check it out!