I have successfully upgraded our home network. I went from an 802.11n to an 802.11ac wireless router. I turned the old wifi router into an wireless access point via powerline ethernet. It took me a while to find the instructions I needed to make a LAN-to-LAN network.
You have to do four things to the old router. 1. Turn off NAT, 2. Turn off DHCP. 3 change the IP address to at least +1 of the main router’s IP last digit (going from say 192.168.1.1 to 192.168.1.2). 4. Plug the ethernet router from the LAN port of the primary router to a LAN port on the old router. Do not plug it into the Internet port. If you do, when you change the IP address, you will not be able to access the web interface of the old router. I learned this the hard way.
I no longer need the wireless extender. I have good coverage in the two rooms I thought I might need the extender for now from the new wifi router. In fact I really only need the new wireless access point in the room I put it in.
I will probably upgrade our cable internet speed though it is quite fast enough for even 4k streaming video, but I’m in no rush as I don’t have a 4k TV yet.