VNXe3200 - unable to upgrade due to storage pool too full

Hi all

Just wondering if any one has encountered this before and if there is a work around?  I am trying to upgrade my VNXe3200 to the latest software (3.1.1.5395470).  I ran the health check prior to starting and there were no issues with the system.  I then commenced the installation process and the health check fails with an error.  Support tells me that my storage pool is too full and I need to delete some LUNs - this isn't actually possible, I have a single thick LUN with no snapshots on the storage pool that consumes 99% of the space.  When I discussed it with live chat engineer they said the LUNs allocated had to be under the pool High Water Mark of 85%.  I have never seen this in any documentation and check the best practises guide there is no mention of a HWM - in fact it says allow 5% for snapshots and 10% for FASTVP (we don't use either) - both are a long way from 15%.  When the system was sized with our account team no mention was made that additional space is required eg: I requested a 42TB usable system and that's what I got, whereas if I have to allow for 15% freespace I now only have a 35.7TB usable system.

thanks

Steph