What do you mean by "migrate"? Do you mean just move your old email into a regular gmail account? Does Settings->Account and Import->Import via POP not work? See eg [1]. Or use Google Takeout and selectively re-import them using, e.g., Thunderbird. (G Apps for Education has a special Takeout Transfer [2] that does this automatically, they could have enabled this for the old GSuite accounts, but it has security implications that are hard to deal with.)

[1] https://www.lifewire.com/move-or-copy-mail-from-one-gmail-ac... [2] https://takeout.google.com/transfer