will i have to set up my family network from scratch with new router
Yes. You configure the router to gain internet access and security. Then you change the settings on the other machines ( wifi ) to match. Or if you can use the same SSID and WEP or WAP code, it should go with out any changes.
i revert back to my lynsky router will i need to alter any settings for the new isp when its up and running.
You will have to alter the settings for the new ISP, but if you leave the SSID and security settings the same, they should connect.
Any machines connected by lan will connect to either, straight away, when you have put in the new ISP settings.