Are you re doing the database too? Food for thought, instead of migrating (assuming you mean grab all users out of a database and insert into another) just use the table already set.
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run export on old db - import to new db (one for one column)
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use old DB
Otherwise there is no "easy" way of doing it. You will have to code a script/laravel command etc that maps the data you want from old db transform as needed to new db.
Off topic: The issue your asking about was to be solved by using the repository pattern as it was marketed. But never solved the issue your question id :)
Least of the worries was a hand full of model classes, it's the data transfer and setup that is the killer. Imaging a million rows of some money column in inadvertently being rounded... Potential millions of $$$ lost (Actually seen it, over 6 years, old dev used float on new db and script didn't account for $19 versus $19.00)