diff --git a/db/data_migration/v2.12/README.md b/db/data_migration/v2.12/README.md
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+++ b/db/data_migration/v2.12/README.md
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+# Migrations in 2.12
+
+This migration updates the configuration of GBV instances *only*. This migrates the fields that calculate average scores on subforms to new versions which support decimal places.
+
+# Verification of data to be updated
+
+These scripts will update the `Field` records where calculations exist.
+
+To validate which fields will be updated, run the following:
+
+```bash
+rails r ./db/migrations/v2.12/update_calculated_avg_fields_gbv.rb
+```
\ No newline at end of file
diff --git a/db/data_migration/v2.12/update_calculated_avg_fields_gbv.rb b/db/data_migration/v2.12/update_calculated_avg_fields_gbv.rb
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+++ b/db/data_migration/v2.12/update_calculated_avg_fields_gbv.rb
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+# frozen_string_literal: true
+
+save_records = ARGV[0] == 'true'
+
+def fields_with_old_avg_calculations
+  return unless PrimeroModule.exists? unique_id: PrimeroModule::GBV
+
+  avg_fields = Field.where("calculation -> 'expression' ? 'avg'")
+  # The new type of calculated average field has the avg object as a hash, not an array.
+  avg_fields.filter { |f| f.calculation.dig('expression', 'avg').is_a?(Array) }
+end
+
+def migrate_field(field, save)
+  old_avg_data = field.calculation.dig('expression', 'avg')
+  field.calculation = { type: 'number', expression: { avg: { data: old_avg_data, extra: { decimalPlaces: 2 } } } }
+  field.disabled = true
+  field.type = 'calculated'
+  if save
+    field.save!
+    puts "Updated field #{field.name}"
+  else
+    puts "Would update #{field.name} to the following"
+    puts field.inspect
+  end
+end
+
+fields_with_old_avg_calculations.each { |f| migrate_field(f, save_records) }