IDA "Corrupt" Databases

Does anyone know how IDA determines that a .idb database file from a previous version of IDA is "corrupt" and refuses to load it. It is obvious that the creators of IDA know that there are non-legit copies of IDA floating around, but how is the registration information for each copy embedded into the database file? Does anyone know what exactly is the program looking for in the .idb file to flag it as corrupt? All I want to do is migrate a few old database files to the newer version of IDA.

xtx