Energy modeling of unrenovated building part of major renovation

We are computing energy performance for a major renovation project in the context of Core&Shell. Modeling the proposed building model we are realizing some difficulties modeling the total building as demanded in LEED 2009 MPR. The major renovation is carried out for approx. 70% of the building. The 3 basement floors and the first floor of the scyscraper building will persist unchanged. The narrow side of the building façade will persist unchanged too. All floors above the first floor shall be renovated (HVAC, exterior wall, windows, roof, lighting). The building standard of the 50 years old building (HVAC, construction, lighting) is mostly unknown. Documentation is very fragmentary and examination of the whole building is not very practicable. So how should we model the HVAC, construction, lighting of the remaining part of the building which persist unchanged. 1) Should we use HVAC of basement building ? 2) Should we use construction of basement building ? 3) Should we use lighting of basement building ? Is there any hint in ASHRAE 90.1-2007 App. G or the LEED Core&Shell guide ? Has there been similar discussion in this board before ? Thanks in advance.