Storage motion impossible during pool upgrade

Hi! I've tried to ask via XAPI's github project [1], but there has been no activity on issues there in more than one month, so I'll try here too.

Initial situation:

• A pool of hosts (any 7.x version of XenServer).
• VMs residing on local storage

Then we want to upgrade the pool to 7.6 without any downtime, so:

• we migrate the VMs from master to another host 
• we upgrade the master host with the 7.6 installer ISO image 
• we reboot master 
• then we want to migrate the VMs back to master so that we can upgrade another host, but we can't: "This operation is not supported during an upgrade"

At first sight, this doesn't look like a bug. There's a commit that explicitly removed `migrate_send` from the list of available operations a long time ago [2].

I'm trying to understand why, so here are my questions:

• Is it wanted, and if so why this restriction? Especially knowing that inter-pool storage migration from an older version of XS to the latest is possible.
• What's the proper way of upgrading a pool of hosts that have VMs on local storage to avoid any downtime due to suspending them for upgrading the host they run on?

Thanks!

[1] https://github.com/xapi-project/xen-api/issues/3764

[2] https://github.com/xapi-project/xen-api/commit/6760dad3cd17576c836efcc17d1b481a9e6f8c77