Ah, thank you for your kind feedback - I'm very glad to hear you enjoyed TROS and I hope you find this game as entertaining.
And to answer your question, you will only need to buy the game once. Once you have purchased it, all future updates (be they bug fixes or new routes) will be available to you. All you will need to do is re-download the updated copy of the game when it has been released.
Many thanks for your interest and I hope you enjoy the routes released so far (Leopold is next!)
PS I will pass your thanks on to Mum ^_~