2 years ago, DC was notified on 5 September and their college allocation was changed, so I think you should be prepared for this possibility. It all worked out very well though in the end. At the time, another poster was frantic and in the end her DC was confirmed at her initial college allocation, so it can go both ways.
It is complicated at Durham - and can't really be compared to confirmation in halls at other unis - because they are trying to achieve a balance of subjects across all the colleges, and within each subject and in each college they want a balance of sex and background and probably grades and maybe other things. They will have achieved this balance in the initial allocation, but post results they need to even everything out again eg in an extreme example, all those with English offers at College X may have achieved their offer and accepted, whereas all the Chemists fell short and they won't double the English cohort and have no Chemists in College X, so there has to be some shuffling around. In a regular hall of residence this wouldn't happen. You might ask why they do the initial allocation at all.
It does seem to take them a while to do this and one would have thought they could write a simple programme to do it, but they seem to go through everything in person.
They are also a relatively small university (only 1/5th of the students of Manchester) so they have a much smaller admin operation and take more time at pinch points like this.
One upside of their small operation, is that they are very slow to bill for accommodation and fees - 1st December!
Despite the relatively late confirmation, the drop-off system worked very smoothly. There were lots of available slots spread over a few days with lots of Freps to help with luggage, so I really wouldn't worry about that, and I'm sure you would be able to make a bespoke arrangement directly with the allocated college if necessary.
Good luck with it all.