BUG: foobar fails when baz about flux HOT 1 CLOSED

As you're probably already aware, flux:field.file will not work the same on TYPO3v12 as it did on earlier TYPO3 versions. Other issues describe the reasons for this in great detail. The TL;DR is: Flux cannot recreate compatibility for this field type which was forcibly removed from TYPO3 itself in version 12. You must find an alternative.

Is there a migration from flux:field.file to flux:field.inline.fal?

There is not, at least not an official one and no unofficial ones that I'm currently aware of. You're far from the first person to have this problem but I don't know if anyone else has solved it with a migration and been nice enough to publish the migration script they used.

There is an update script for TYPO3 10, but not for 12, right?

Correct; that upgrade script however is only dealing with a much older Flux change and was only needed a couple of major versions and many years ago - it rewrites colPos values. It does not change file references at all.

Does it generally make sense to change the whole thing to flux:field.inline.fal?

That field type is as of now (TYPO3v12 and future) the only one that will work to let you select a file as a sys_file_reference so that is the recommended approach. I know of others who tried using an inputLink field type with files as only link target type but this comes with multiple drawbacks (for one, it requires parsing of the field value to extract a UID of a file and AFAIK, will not support cropping or scaling of any kind).

If you go with flux:field.inline.fal or generic flux:field type="file" (which are the same thing on v12) see also #2139 (comment).

from flux.