Hi,
We've done several large projects with jBPM (mainly version 3) and from a technical perspective I don't see problems of replacing jBPM with Activiti. What's important is to determine if just plain jBPM is used at your customer or if they have also developed custom functionality, like workflow extensions or something similar.
And what's also a challenge is to migrate the jBPM database to the Activiti database. In the next release Activiti will provide support for database migration.
So it's quite difficult to say something about this specific customer case, but in general it should not be complex to do a migration.
Best regards,