Inbound or Outbound

My condo is planning on redoing the plumbing. Both inbound and outbound plumbing need to be replaced, but at this time they must make a choice. Currently, drainage water backs up into some lower units
due to bad drainage pipes. They are planning on just replacing the inbound plumbing at this time. If the current drainage pipes can’t handle the current water (volume and pressure) do you think, replacing the inbound pipes resulting in higher water pressure will cause even more of a strain on the drainage plumbing resulting in more frequent back ups into lower units?
-Seth