docs(studies-upgrade): ensure upgrades only for raw studies without v…

…ariants

docs/how-to/studies-upgrade.md

@@ -2,6 +2,7 @@
 title: How to Upgrade a Study?
 author: Laurent LAPORTE <[email protected]>
 date: 2023-03-10
+revision: 2024-07-03
 tags:
 
 - upgrade
@@ -31,6 +32,10 @@ hesitate to contact our support team for assistance.
 
 ## Upgrading
 
+> **WARNING:** Upgrading a study is only possible if the study is a **raw study** without any variants,
+> as it is not possible to update its descendants. Also, upgrading a variant study is not possible.
+> If you have a variant study, you must first create a new raw study and then upgrade it.
+
 To upgrade your study to the latest version of Antares Web and Antares Simulator, you can follow these steps:
 
 On the main page of the study, you can find the version number at the top of the menu bar: