If you have myCloud Pro and cancel your subscription it will be changed to myCloud Standard or myCloud Light (if you have an authorized Swisscom subscription it will be changed to Standard, otherwise it will be changed to Light). Your storage will be reduced to the corresponding amount. No data will be deleted even if you have used up more than the allowed amount of storage. However, in this instance, you will no longer be able to upload any more data.
If you have myCloud Standard and cancel your subscription it will be changed to myCloud Light. Your storage will be reduced to 10 GB for data, photos and videos. No data will be deleted even if you have used up more than 10 GB of storage. However, in this instance, you will no longer be able to upload any more data.