Thanks. I can see the support ticket, but can't make changes to it, so I'll just suggest a workaround here (also as a benefit to anyone else who might come across this thread): You might be able to work around this issue by killing the the GemFire 6.6 members rather than shutting them down using shutdown all, and then attempting to upgrade their disk stores.
I will email the support engineer to bring him up to date.
