Generally each replacing the wiring and refrigerant pipes will need to be replaced. The older the unit the refrigerant gas, required pipe installation and copper pipe thickness needs to be changed over sure to newer systems not being compatible with the current systems. This is at times costly however overall a good thing to replace. The existing system is likely to be running an ozone depleting HCFC type gas which has been phased out in the industry and governments. The pipe insulation thickness may have inadequate or have a thin wall thickness. The thinner the wall thickness the more efficiency can be lost in the refrigeration cooling/heat removal process.
Replacement of existing wiring is generally required due to newer systems having different wiring/ sizing requirements. This can be from the indoor to outdoor unit interconnecting cabling and also the power supply from the property switch boards dedicated AC circuit to outdoor unit.
Current wiring codes require all dedicated ac circuits to wired into switch boards to have a residual current circuit Breaker with overcurrent protection (RCBO) type breaker. This is the current electrical code requirements with majority of installation over 3-4 years not having an RCBO breaker installed within the property switchboard