Hi,
The jconn3.jar is in the jConnect 6 folder which is listed in the above link for the Sybase Online books.
jconn4.jar is in the jConnect 7 folder.
So ultimately the customer will have to test those older applications to the newer ASE. If they run into problems please report them. I might be they have to set a connection property to make sure it works correctly. Here is a list of the jconn3 (jConnect 6) connection properties for further reference: SyBooks Online for jConnect connection properties