CUSIP is a 'header' variable is backfilled to show the new or latest number throughout the entire time series. This is how COMPUSTAT and CRSP set up their databases (i.e. the characteristic is not WRDS specific). This feature is similar to GVKEY (the permanent company identifier in Compustat) and PERMNO/PERMCO (same purpose in CRSP). When a data set is updated it is possible for that company's CUSIP designations to change. Except under extraordinary circumstance, GVKEYs and PERMNO/PERMCO stay the same.
NCUSIP for 'NAMES' CUSIP in CRSP reflects historical designations. Compustat does not carry a similar historical CUSIP item
There are various threads in WRDS Forums that discuss CUSIP merging issues. The main problem is that the GVKEY-CUSIP pair can change when a new Compustat vintage is produced, even for back years.
And while CRSP's CUSIP variable is similar: it is matched one-for-one to each PERMNO in each CRSP vintage, CRSP provides a historical NCUSIP file that we match to PERMNOs in a historically accurate fashion. In other words, CUSIP is backfilled and not necessarily historically representative of the security ID, while NCUSIP solves this problem.
Our web queries for the CRSP daily and monthly stocks allow you to include NCUSIP as the Historical CUSIP, and ignore the header CUSIP. We also have sample SAS programs to merge NCUSIP to the stock files (DSF or MSF datasets) if you prefer to do it yourself. There are also CRSPAccess and FORTRAN utilities that can perform similar tasks.
Unfortunately, Compustat does not provide a historical NCUSIP set to perform a similar task.