Could not move this URL to a different zone. The primary Url for the site collection must stay in the default zone

I have a SPSite with 2 URLs.

http://site1.domain.com (default)

http://site1. (intranet).

I want to flip these. I want to make http://site1 as default zone and http://site1.domain.com as intranet.

How can I flip the URL for the default zone and intranet zone.

this is for Host Named site collections.

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