Not sure if there is a simple means to migrate these fields. Have you seen Deprecated nFeed field - Valiantys Wiki?
That page would seem to indicate that there might not be a need to migrate these fields, as they still do appear to be supported by the plugin vendor. From that page:
Why didn't you directly migrate this field ?
We wanted to make sure that there are as little impacts as possible for existing configurations. There will certainly have minor changes or bug fixes in next version and we wanted to start the migration on the most stable version possible.
The latest release includes a tool that migrates Deprecated fields to Standard fields automatically, check out the documentation -> https://valiantys.atlassian.net/wiki/spaces/NFEED512X/pages/99778599/Deprecated+field+migration
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