Could not ever find any official documention on this. I really wish NetApp would publish something on this topic, some KB or something!!!
What I have noticed/done in the past is simpley delete the aggregate. As you know MDV volumes are placed on every aggregate when auditing is enabled. Even a new aggregate after the fact gets the MDV volumes added. I am believeing that once a volume is migrated off an aggregate, the MDV on the prior aggregate is no longer used on only updated on the current aggregate. I also suspect that it may not necessarily move auditing information during the volume move. Hence, delteing the aggregate may in fact remove some historical audting information.
With that said, I cannot recall, but it did require using diag or at least advanced mode. I was able to forcably delte the MDV volumes on the aggregate I was removing. After that, the aggregate removd easily.
Of course, I may have had to use the force method on the aggregate as I could not remove the MDV. Again, I do not recall completely.
Either way, I was able to use the "-force" flag and either : delete the MDV and the offline/delete the AGGR, or: using the "-force flag" I was able to offline/delete the aggregate. I suspect it was the first.
Again: IF SUPPORT IS WATCHING: You should really come up with a KB article documenting how to properly remove a MDV volume from an aggregate that is being decommissioned and that process should not even come close to including: turn off auditing!