required dates to be postponed

I made a with msc cruise south Africa to travel 09Feb 2018...Then I received an email of the cruise being a themed cruise now and if I don't wanna stay on the boat I need to send them a request to move or pay an additional R1300.00...I phoned and asked is it compulsory to pay the R1300 and they advised yes so I opt to move.....they are not willing to give me a suitable date neither a cancellation....they guy I spoke to said I must give him dates...wanted to move to Nov 2018 he said they cant move me to Nov its out of the season I booked,.....wanted to move to end of March...they cant cause its peak season.....then the guy said to me I can stay on the cruise and I don't have to pay...when I said I want something in writing to confirm he is ignoring me now,,,,Please assist they are the ones that added this extra change now I must be the one to be inconvenienced..I wanted them to move me or refund me