The only area I think you might have problems would be behind the toilet. The only reason you would need to go there is if they was split pipe there. I took all my old plumbing out and replaced it. It goes behind the tub under the bed by the bathoom under your sink to the pump. your valves should be in the aft end where the door is, the hot water heater and under the sink.
I was totally freaked out re-doing the plumbing, but once I went with pex pipe. it went pretty smooth. There are pics in this forum that show you how it is done. I used those pics to do mine.
Brian & Adrienne