Is there a simple way to increase the maxlength value of a plain text field that has data in it already?
To directly answer the question, there is no simple way in Drupal 8 to modify the schema for a field once it's been populated with data.
But, there is a more complicated way to manage it.
We've used the work done here by J-P Stacey to update a field in our platform.
- ALTER the database tables directly using raw SQL.
- Change the site configuration for the field.
- Force the current, "desired" schema records in Drupal to match.
- Force the "most recently installed" schema records to also match.
Additional information can be found in this article written by Marcus Bernal.