Know if I need a permit to replace the pipes which drain the sinks and toilets to the sewer system?

Yes, permit is required for all sewer connections. Proper materials and installation are important for the smooth operation of your drain and wastewater system. Proper piping prevents your potable water system from contamination. 

SAFETY FIRST - Some people with experience and knowledge can safety replace minor plumbing fixtures insuring water and sewer pipes are not cross-connected.