Mirating mobile clients to auth0 and avoiding re-login

In order to migrate to auth0 without requiring our users to re-login, we’re looking for some way to convert an existing legacy authentication server access token to auth0 access/refresh token. Basically, on startup the app would call a token conversion endpoint and would get auth0 access/refresh token back.
One approach that seems like it might work is to create database users with a known password, and use the ROPG to get the tokens.
Do you have any other recommended solutions for this? Thanks!