A company PERMNO can change simply because PERMNO is a security (stock) identifier and companies can have more than one stock at one time and then decide to retire an issue.
Companies may also re-organize and issue new stock that old stockholders have no claim upon.
For example, PERMNO 16707 is used for the MANVILLE CORP (date range: 19280229-19881130) before a reorganization in late 1988. The new PERMNO is 90100 for MANVILLE CORP (date range: 19881130-20010228), which the CRSP database also describes as JOHNS MANVILLE CORP NEW.
If you extract DELISTing codes for any PERMNO from the MSE event's file you can look at NWPERM to see how CRSP handles different cases of reorganization. Some are hard to follow and an assigned of PERMNO may be changed by CRSP in different database vintages. To verify any particular case, send email to CRSP-Support ( email@example.com )
You also need to be careful when using Ticker Symbols and how you translate TICKER to PERMNO. This case shows the 2 companies used at least three different tickers, with JM used by both the old and new JOHNS MANVILLE CORP.