That's not a problem, fortunately. You just need to build the bond before you run your 'vsx_util reconfigure'. What would be a problem is if you were using eth3-## interfaces directly. The 7000 only has two card slots. If you're using interfaces which cannot exist on the new box, you will have to use 'vsx_util change_interfaces' first to move to interfaces which can exist. The ideal VSX deployment has everything in bonds, as bonds are easy to move between physical ports on the members.
If you're going to a different version (I don't know if the 7000 can run R80.30), you will have to use 'vsx_util upgrade' on the management to update the object definitions to the new version.
If all of your interfaces can exist on the new boxes, you need to make your bonds, run through the initial config (either via web UI or config_system), shut down the member you are replacing, use 'vsx_util reconfigure' on the management to have the new physical box take over the old object, then apply any dynamic routing configuration to the VSs. The process is then repeated for the other member.
If all the boxes are running the same version, you can also add the new members to the cluster with 'vsx_util add_member' and remove the old members from the cluster with 'vsx_util remove_member'. This is commonly done if you have hardware information in your hostnames.