chore: migration guide changed resampling method (#38)

docs/docs/about/migration_guides/v8-1_to_v8-2.md

@@ -13,3 +13,20 @@ sidebar_position: 0
 ### operational settings used is now 1-based
 
 Consumer systems will now refer to the first operational setting as 1 instead of 0. Zero means "No setting used".
+
+### Resampling of rates changed from forward filling to average rates
+
+All calculations are performed on a global time vector, which is the union of all dates found in the 
+input resource files (csv files) where [INFLUENCE_TIME_VECTOR](../references/keywords/INFLUENCE_TIME_VECTOR) is set to 
+True, dates found in the eCalc model yaml-file (temporal models), and dates in the requested output frequency.
+
+If the global time vector and the dates in the requested output frequency does not coincide fully, a resampling of the 
+results needs to be performed. Previously this was done by simply picking the first 
+available rate in the time interval (forward filling). The rates are thought to be constant in a period between two 
+dates, hence the forward filling will disconnect the rates and the cumulative volumes. This has now been changed to 
+calculating the average rate from all dates in the global time vector within a date range in the requested output 
+frequency, to keep the rate and cumulative consistent with each other. This average will take into 
+account the lengths of the periods and the regularity within each period. The figure below shows a comparison of how the
+resampling would previously have been done compared to how it is done now when making quarterly output from monthly results.
+
+![](/img/docs/changed_rate_resampling.png)