Plumber changed plan without telling me

[Carl Hewitt]

We had discussed replacing badly damaged plumbing in a slab home and running the new pipes outside through the yard until it meets the city line in the front yard instead of going through the floor in home. I received quote of $14k which included extra costs due to a tree nearby they'd have to navigate. This is in South Florida. 

They submitted plans to the city like we had discussed, but when my manager was there he noticed no digging in the yard. He found the revised plans which now showed them placing new 3" pvc inside the old 4" steel pipes. I was never notified of this. 

My issue is that this seems like it should now be a much cheaper job. I already paid for most of the work, since I didn't know there was any change to the original plan. My manager asked the plumbing tech, and he said he was told it should be about the same cost when he asked about it. Does anyone have any thoughts they can share?