docs: add missing migration guide change (#768)

equinor · Jan 22, 2025 · 5122564 · 5122564
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commit 5122564
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diff --git a/docs/docs/about/migration_guides/v8.20_to_v9.6.md b/docs/docs/about/migration_guides/v8.20_to_v9.6.md
@@ -10,6 +10,18 @@ This migration guide is intended to help you migrate your existing eCalc™ mode
 is divided into 3 sections; **v8.22 to v8.23**, **v8.26 to v9.0** and **v9.0 to v9.6**. Make sure to go through the
 whole guide to make sure you have updated your model(s) correctly.
 
+## v8.20 to v8.21
+
+### 1. Changes to EFFICIENCY
+
+We no longer allow 0 efficiency in input chart.
+- `EFFICIENCY` for `FRACTION` must be between 0 and 1, <0, 1], greater than 0, and less than or equal to 1.
+- `EFFICIENCY` for `PERCENTAGE`must be between 0 and 100, <0, 100], greater than 0, and less than or equal to 100.
+
+### 2. Changes to PRESSURE_DROP_AHEAD_OF_STAGE
+
+`PRESSURE_DROP_AHEAD_OF_STAGE` for Simplified Variable Speed Train has no effect and is now treated as an error.
+
 ## v8.22 to v8.23
 
 ### 1. Changes to COMPOSITION