Bug 799592 - "No transactions found" when importing CSV transactions

Before this when changing the base account to one with a new currency
the new currency was added to the old base account's currency in the
alternate currencies vector instead of replacing it.

To avoid that re-parse the the splits from scratch when replacing the
base account.

gnucash/import-export/csv-imp/gnc-import-tx.cpp

@@ -182,6 +182,7 @@ void GncTxImport::base_account (Account* base_account)
         return;
     }
 
+    auto base_account_is_new = m_settings.m_base_account == nullptr;
     m_settings.m_base_account = base_account;
 
     if (m_settings.m_base_account)
@@ -192,10 +193,17 @@ void GncTxImport::base_account (Account* base_account)
             set_column_type(col_type_it - m_settings.m_column_types.begin(),
                             GncTransPropType::NONE);
 
-        /* Set default account for each line's split properties */
-        for (auto line : m_parsed_lines)
-            std::get<PL_PRESPLIT>(line)->set_account (m_settings.m_base_account);
-
+        if (base_account_is_new)
+        {
+            /* Set default account for each line's split properties */
+            for (auto line : m_parsed_lines)
+                std::get<PL_PRESPLIT>(line)->set_account (m_settings.m_base_account);
+        }
+        else
+        {
+            /* Reparse all of the lines with the new base account's commodity. */
+            tokenize(false);
+        }
 
     }
 }