I would advise upgrading to the most recent version available to your customer. Almost all changes to SAS/IML are designed to be source-level backward compatible with existing releases. That said, SAS itself has changed quite a bit since V6 of IML, and I can't guarantee that there won't be some inconsistencies. The output may look cosmetically different, because current SAS/IML uses the SAS ODS system, which V6 did not. Aside from that, the basic functionality that was as in V6 is still there, plus A WHOLE LOT MORE, including SAS/IML Studio.
Regarding SAS migration/upgrade, here is a good place to start. Also, consider that SAS and its components have come a long way since SAS Version 6, so your enterprise/organization should really be more active with maintaining a more "current" and "vendor supported" production SAS environment, if considered mission-critical.