VM live migrations fails from 6.5 SP1 to 7.1 when VDI type=User

We have a fresh installation of XenServer 7.1 and currently moving existing 6.5 Pool VMs to 7.1 Pool.

Majory of VMs live migration works but some fails (even with force=true) with following error: 
The VM could not be imported.
msg: INTERNAL_ERROR: [ Http_client.Http_error("500".....
Found XSO-714 issue and tried the "vif destroy solution" with no luck.

After some troubleshooting, found the following:

• If VM attached VDI has type=User, live migration fails
• If VM attached VDI has type=System, live migration works as expected

I'm unable to change VDI type and I'm not 100% confident of this parameter. Tried the following in a sandbox VM with no luck:

xe vdi-param-set uuid=xxxxx type=System
Error: Field 'type' is read-only.

How can I solve this issue? 

Regards,

J