Any way to use on existing fields?

This is fantastic. Was looking for how to convert existing media fields to media with contextual modifications!

A note for others who may make the same mistake i made. If you get:

[ERROR] Not an entity reference field

[OK] Migration complete.

And run you are quite certain the field is indeed an entity reference field, check that you are indeed first putting the entity type, not the bundle, so for example if you have a field on the page content type you want to run drush media_library_media_modify:migrate node field_image and not have 'page' instead of node; field storage can be on multiple node types not only on one at a time, that's why this works this way.