I used the following command to migrate the only conferencing attendant, and result seemed OK
A detailed procedure is available in this excellent blog: https://www.simple-talk.com/sysadmin/unified-messaging/migrating-from-ocs-2007-r2-to-lync---part-3/
To confirm that the number was migrated to the new pool, I checked the Lync Server Control Panel; dial-in entry was there, but got a warning stating no region was assigned to it.
When trying to assign the region via the Lync Control Panel I got this error:
Set-CsDialInConferencingAccessNumber : The region mappings for this access number are inconsistent.
I was unable to find any public resource with the error above.
Resolution was to assign the region manually via Powershell and using the -ScopeToGlobal switch