Replying to myself... one idea... I sent the forms to a "middleman". He tested the forms, edited, saved, etc to make sure they worked. If he sent them his previously edited and saved files (like he edited and saved for testing, then deleted his edits and saved again) instead of my original unedited files... would that cause the issue and disable the extended features for the end user? Should he have forwarded on my original files? or does it matter? maybe not... just a thought based on the error sentence "the document has been changed since it was created..."
That might have caused it, yes, depending on what application they used.
But the bigger issue is why do you apply these rights at all? Assuming that the users have a relatively new version of Adobe Reader it's not necessary, as it can do all of these actions you mentioned by default.
If all users have the version of Adobe Reader they need to do this function, then they wouldn't be having the problem at all, and I wouldn't be having to figure this out. I would love that. We cannot assume that the 200 users have a specific version of anything consistent with each other. The rights were applied per request. Maybe there's a better way. I'm all ears. For now, I'm looking for a reason or a work around for those that are affected with the error/disabled rights.