Hi @lee-martin
Yes it is possible to change the site url.
Steps:
1. Log into https://make.powerapps.com
2. Click apps > select your portal app >. Three dots > settings> administration
3. It will open up the powerapps portal admin center
4. Go to Portal Actions > Change base URL.
5. In the dialog box enter the new url and click ok.
Note: it is also possible to implement the custom web domain
Hope it helps.
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