First of all, the pipeline into the house is already determined by the architectural design, they can not be replaced privately, because they have to consider the pressure and construction factors, which will affect the use of other households.
Secondly, the replacement of the pipe and the installation of water purification system is not related to the installation of water purification system, the outlet is not possible to go back to the original pipeline, but the water purification system comes with the pipe, you can place the original pipe can be.
Finally, you can make slight adjustments according to the actual use and installation without replacement.