Where the identity of the transaction on which plaintiffs based their claim to a judgment against the defendant and the form of the action as one sounding in contract were preserved in the amended petition, and this is the best and most reasonable test by which to determine whether or not an amendment of a pleading substantially changes the cause of action so as to be obnoxious to the rule against departure, the amendment of the written contract does not constitute a departure. Stewart v. Van Home, 91 Mo. App. 647; Sain v. Rooney, 125 Mo. App. 176, 101 S. W. 1127.