It is easier to change the access point's address before adding the new gateway to the network, or the addresses will conflict. To change it after the new gateway is in place you would should separate your computer from the network and attach it only to the old router, or you may wind up reconfiguring the new gateway by mistake. I actually have another 192.168.1.xxx subnet isolated from the rest of the house that I can use while configuring customers' equipment
The typical home network will be:
192.168.1.1 main router (gateway)
192.168.1.2 access point
192.168.1.3 another AP if needed
etc.