Issue with hard drive upgrade and xraid

I'm not sure what you mean by "the volume is not expanded".  Do you mean that the volume didn't expand at all, that the volume didn't expand to the additional space on the last drive, or what?  The NAS certainly believes your volume is expanded, and I suspect it did expand when the first few drives were swapped with XRAID enabled..  By "the last drive", do you mean the last of all 8, or some number less?  If something less, that's the expected behavior of FlexRAID.

As for that message, unfortunately, you gave bad advice.  What she is seeing is simply "the way it works".  It wasn't always the case, but it's been the case since at least 6.9.something when I ran into this same situation.  If you have a multi-level FlexRAID volume, switching (back) to XRAID is not allowed.  I don't understand why this is the case since, AFAIK, all FlexRAID expansion is exactly as XRAID would have done it, there are just fewer options.  It's clearly not a bug since the message correctly tells you what's going on, so programmer laziness seems most likely.  While I also had an EDA500 at the time I discovered this, I don't believe that's a part of the issue.  I recall I ran some experiments proving otherwise.

If it really didn't expand (completely) and all drives were replaced, then perhaps StephenB can help with that.

Unless somebody at Netgear who is familiar with what the OS is looking at to prevent the switch and how to work around it gives some advice here (and they didn't a few years ago when I first reported this behavior), her only solution to get back to XRAID is to destroy and re-create the volume.  There may be no need to do that now, but it will be necessary if she ever wants to increase the size of less than the full complement of drives.  That she has an EDA500 may create some issues even when doing that, as the NAS may want to convert the EDA volume to the primary volume (which is a really bad idea).  When I took the plunge and did this, I first exported the EDA volume, then destroyed and re-created the main volume, then imported the EDA.