You can actually do this, works fine for me, although it's not officially supported or recomended.
I recently upgraded to a Superhub 3 (from a superhub 1) - they did not want the old superhub 1 to be sent back so it's basically scrap, its only usefulness is the potential to use it as an additional wifi access point to an existing network.... if your old superhub is a 2 or a 2ac they might well want it back, but can't comment on that as I never had one.
You never said which specific equipment you are trying to configure, the example I give is based on my own setup that contains a superhub 1 and a superhub 3.
Apologies that I don't have time right now to provide detailed step-by step, but what you basically need to do is outlined below:
1. while still connected only to the old hub, change the LAN IP address to a different address, I would suggest 192.168.0.2 (The default is 192.168.0.1). You should also disable the firewall and disable the DHCP and disable the NAT routing, as all of this functionality will be handled by your new hub acting as the gateway.
2. disconnect from old hub and then connect to the new hub and make sure the DHCP address range does not include 192.168.0.2 as this is the fixed IP address that you have just given your old hub.... I personally have a DHCP range that starts from 192.168.0.10 which means IP numbers 2 to 9 are effectively reserved for non-DHCP fixed IP addresses (I think this may even be the default behaviou on the hub3)
3. then simply connect the two hubs together with an ethernet cable to any of the ports on both hubs (they will need to be connected with a cable, it won't work as a wifi-relay, only an access point from an ethernet connection).
4. the old hub (now only acting as a wifi access point) should automatically find a route to your gateway... I'm not sure if it makes a difference but I would make sure you always turn on the power to the main gateway hub first and the old hub last, because it may not find the route to the gateway if your new hub is not already fully up and running. n.b. this is the only iffy bit of the config because you can't manually set the gateway address of the superhub because it would normally assume that the gateway is itself.... but it does seem to work somehow. - You should now be able to configure both hubs separately by connecting to either 192.168.0.1 or 192.168.0.2 - if you use the same SSID and password for both superhubs then your devices will connect to whichever one has the strongest signal.
