Change field settings for text field from plain to formatted while data exists
Attention for Drupal 8:

You need to update at least 3 or 4 yml files

field.field.node.page.field_description.yml

(same change)


field.storage.node.field_description.yml

--- a/config/default/field.storage.node.field_description.yml
+++ b/config/default/field.storage.node.field_description.yml
@@ -4,13 +4,13 @@
 dependencies:
   module:
     - node
+    - text
 id: node.field_description
 field_name: field_description
 entity_type: node
-type: string_long
-settings:
-  case_sensitive: false
-module: core
+type: text_long
+settings: {  }
+module: text
 locked: false
 cardinality: 1
 translatable: true


core.entity_form_display.node.page.default.yml

-type: string_textarea 
+type: text_textarea


core.entity_view_display.node.page.default.yml

-type: basic_string
+type: text_default


Maybe the last 2 ones you can deal with those changes on UI(go to each teaser and save). But I did in config folder.