It may still be possible to make changes to your return train ticket if you decide to stay an extra night at Aguas Calientes. You will have to take your train ticket personally to the train station in Aguas Calientes and ask them to change the return date of your ticket. You will probably be asked to pay an extra administration fee which will be dependent of the train service and departure time and changing the ticket will be subject to availability of spaces on the train on the following day. Before doing this you should also check at the Machu Picchu ticket office in Aguas Calientes if there are any entrance tickets to Machu Picchu available. During the peak season entrance tickets can sell out 4 or 5 days in advance. Unfortunately if you decide to change your train ticket yourselves we will not be able to provide transport for you from the train station at Ollantaytambo back to Cusco and the bus ticket price US$5 will not be refunded (see info above).