Okay, I did what I should've done before posting a question, and checked the technical manual. It seems getting the cluster out is straight forward. Remove radio, remove temperature controls, remove cluster panel and then remove the gauge cluster. Nissan calls it a "combination meter" for whatever reason. I've attached the relevant page here if someone else comes across this topic in the future.
I haven't found where mileage is stored, but I'm assuming in the cluster itself. I'm gonna order one with similar mileage. If mileage turns out to be recorded somewhere else that would be a bonus, but if not a few extra thousand miles on the odometer won't hurt and won't set off any alarm bells during the next MOT.