Is it OK to replace html on resize?

I was asked to take over a web project. It is supposed to be a responsive website, but the former programmer didn't write the HTML/CSS with responsivity in mind and adding code to make it responsive would be a nightmare. Mobile version on m.domain.ltd is not acceptable by the client for some reason. Is it OK to just replace whole page (ajax call to server) with another content when the window is resized or loaded in, say, <780px? Thanks