hi @Nic44
I'm not sure if you'd be charged the fee's twice (my gut says if you sent an adjustment request to the host, and if they accepted then you probably wouldn't be, but if you just cancelled the days then re-booked second part you probalby wouldn't get them back.
But you might want to be mindfull that even if the host has flexible cancellation, you may want to mention the going away for a part when you book.
It's fair to go away at xmas, i can understand why you might want to, but keep in mind your host may also be away. If you are origonally booked from mid december to jan, it's perfectly understandable for your host to not be available on the 26th/27th/28th etc to re-check you in.
And it's hardly fair of you to keep the keys while you're away if you are not paying...