Use smarter backward compatibility check

WalletWasabi/Stores/IndexStore.cs

@@ -162,10 +162,29 @@ private async Task TryEnsureBackwardsCompatibilityAsync()
 				foreach (var fileName in oldFileNames)
 				{
 					var oldFilePath = Path.Combine(oldIndexFolderPath, fileName);
-					if (File.Exists(oldFilePath))
+					var oldFile = new FileInfo(oldFilePath);
+					if (oldFile.Exists)
 					{
-						File.Copy(oldFilePath, oldFilePath.Replace(oldIndexFolderPath, WorkFolderPath), true);
-						File.Delete(oldFilePath);
+						string newFilePath = oldFilePath.Replace(oldIndexFolderPath, WorkFolderPath);
+						var newFile = new FileInfo(newFilePath);
+						// First, delete target file if exists, as File.Move() does not support overwrite.
+						if (newFile.Exists)
+						{
+							var newFileLarger = oldFile.Length > newFile.Length; // Because during development we may be moving between branches.
+							if (!newFileLarger)
+							{
+								newFile.Delete();
+							}
+						}
+
+						if (newFile.Exists)
+						{
+							File.Delete(oldFilePath);
+						}
+						else
+						{
+							File.Move(oldFilePath, newFilePath);
+						}
 					}
 				}