Increase max length of a text field? 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.