You can always call the call centre and let them know the dates you will be unavailable. I did the same in my case. However the call centre will only amend the dates once. so only if you are confirmed on the dates let the call centre know. If call centre cannot do it you will have to write a letter to the citizenship office notifying the same.Me DM since 30 June 2017. still waiting. I am going on vacation in 3 weeks time for 3 weeks. hope they do not send me oath invitation when I am away