Matt, Thanks!
That worked...It's odd that the title for the UX is Change DOMAIN Admin when in fact we are changing the Primary Admin. It works but could be confusing as there is no reference to Primary Admin.

We've also noticed that Forwarding and deleting from the Primary Admin is hit and miss. Some emails from our test gmail accounts are not deleting or forwarding. But others internal emails are properly forwarding and then deleting from the same account. Known bug?

There is a chance it could be an impersonation bug. To test that theory you can create a temporary password for the primary admin account and logging in and trying that way. If you notice it working but then if you impersonate you see the odd behavior then it is definitely a bug relating to impersonation. I can't really see any other reason why just the Primary Admin would have issue with that as being a Primary/Domain admin/user shouldn't affect how emails are forwarded.

Without impersonating these are the logs from 2 tests I just ran:
You can see the gmail account "Delivered" while the local account (steve@sgdesign.com) was Forwarded and Deleted. I checked the person's Contacts in case there was some tie but my test account is not present in their contacts.  If you like I can still do the Impersonation Test but since I was not logged into the account it may not be any different.

[2019.02.11] 13:32:05.608 [27075] Delivery started for rpohl@isemanhomes.com at 1:32:05 PM
[2019.02.11] 13:32:11.624 [27075] Delivery for rpohl@isemanhomes.com to mjamison@isemanhomes.com has completed (Delivered) Filter: None
[2019.02.11] 13:32:14.624 [27075] Delivery finished for rpohl@isemanhomes.com at 1:32:14 PM    [id:x23054027075][2019.02.11] 14:29:26.717 [19175] Starting local delivery to rpohl@isemanhomes.com
[2019.02.11] 14:29:26.717 [19175] Delivery for steve@sgdesign.com to rpohl@isemanhomes.com has completed (Forwarded Deleted) Filter: None
[2019.02.11] 14:29:26.717 [19175] End delivery to rpohl@isemanhomes.com (MessageID: <45FF2287-CD54-4BC3-B99F-40FDC7225618@sgdesign.com>)

[2019.02.11] 14:29:53.736 [19176] Starting local delivery to rpohl@isemanhomes.com
[2019.02.11] 14:29:53.736 [19176] Delivery for steveguluk@gmail.com to rpohl@isemanhomes.com has completed (Delivered) Filter: None
[2019.02.11] 14:29:53.736 [19176] End delivery to rpohl@isemanhomes.com (MessageID: <6BAEFDE8-2550-4E74-AF21-78136921E0CB@gmail.com>)