This has been on my mind for years and I finally got the time to create a new version of my PowerShell script to re-index user profiles for the scenario where you map a new UPA property to be used in search.
You can pick up reindex-users-v2.ps1 from https://github.com/wobba/SPO-Trigger-Reindex. The script allows for quicker updates especially for tenants with a lot of profiles as it uses the UPA bulk import API instead of iterating the UPA.
See https://docs.microsoft.com/en-us/sharepoint/dev/solution-guidance/bulk-user-profile-update-api-for-sharepoint-online and https://docs.microsoft.com/en-us/previous-versions/office/sharepoint-csom/mt642955(v=office.15) for documentation.
Cover image by Markus Winkler @ Unsplash
