Create a WiFi Mesh System with the NEW Nighthawk EX7700 Mesh Extender

Our NightHawk has really helped with a lot of areas we were not getting strong signals, however I do understand and have witnessed some "bottlenecking" I believe. Our Arris is just in such a poorly placed spot of the house, we've always had our modems in the same room since we had the line(s) from the street ran to one side of the house and into my office which was done 20+ years ago before any of us knew WiFi would be the way of the future.. otherwise we would have told them to run the line to the front/middle of the house.

One day I have time I will most likely change the modes and shut the WiFi on the Arris off, the NightHawk gives off a great signal... but the Arris WiFi is just weak IMO. Our house is not large, I feel like this Arris was built for a house half our size (which would be like the size of an apartment).

I am wondering if I due decide to switch modes to beef up the WiFi downstairs as we have a hardline we could run to the NightHawk, if I could still keep the hardlines (3) connected to the 2 PC's in this office and 1 to the PS4. I prefer to keep my gaming PC's and PS4 Console connected via ethernet cables. It would be pretty impossible or at least very difficult if we switched modes and then I had to run new hardlines from the NightHawk (downstairs) back up to this office.. I'd like to leave the Arris in it's place it has always been and keep these 3 devices plugged into the Arris. I do not care about WiFi upstairs, I'd prefer everyone downstairs get the best of the WiFi, we have 1 PC and 1 Console we could hardline into easily for the area near the NightHawk (where it currently is located in the middle of the house).

We have a hardlined PC right next to the NightHawk that we could plug into the NightHawk once modes are switched, but there will be a lot of empty unused ethernet ports on the back of the NightHawk... as I mentioned our hardlines were all ran through the house 20+ years ago from this office on one side of the house...

I've read the instructions and it doesn't seem difficult at all to switch modes, and I know that the NightHawk is MUCH stronger than the Arris in regards to putting out a WiFi signal as the bars are full all over the house (many places where they were very weak)... I am just worried if I switch modes if I could still keep these 2 machines and 1 console plugged into the Arris... and use the 4th line to plug into the NightHawk downstairs...