Upgrading from stock Linksys WRT1900ACS to OpenWrt WRT3200ACM

I've got a stock Linksys WRT 1900ACS that is pretty satisfactory except it does not have the range I need and only works with two DDNS services. I'm planning to replace it with a Linksys WRT 3200ACM router with OpenWrt. Then I'll move the 1900 closer to the area that's currently getting the worst service, connect it to my LAN, and run it in bridge mode to extend WiFi coveage.

But I want to take my time setting up the replacement 3200, which means I don't want to take my network down while I'm flashing & configuring the new 3200. Can I just change the 1900's IP address to, plug the 3200 into the system, and treat it as just another device (at on the network?

Or will the two routers conflict with each other? If so, what's the best way to work with the two routers simultaneously?

It should work if you connect them between LAN ports, so that they have fixed IP addresses from the same subnet. (This way you avoid any double NAT)

You will need to provide the main router as the gateway and DNS server in the settings of the non-internet connected router.