Working with a IATA Agency I can confirm we received various agent newsletters from airlines stating flights to/from CMB will need to be rescheduled due to runway works.

I am afraid you will need so speak with Srilankan to see what reprotection they can offer you. If nothing suits and you are able to find something with other airlines that work for you schedule and price wise you could also claim a refund I suppose and book somewhere else, but check with Srilankan first.