change price/amount decimal (24,4) in catalog_product_entity_decimal, catalog_product_index_price, catalog_product_index_price_tmp, catalog_product_index_price_final_tmp,
change the MAXIMUM_AVAILABLE_NUMBER in /quote/model/QuoteValidator.php to 999999999999999,
And then change price/amount decimal (24,4) in all quote tables, all sales order tables, all sales invoice tables too..