Think it depends on the attitude of the store people really. I also heard it can be done instore, but my local store wasn't interested, telling me 'that we can't do that, you need to ring in to 789 to get it done)
yet a work collegue who lives in another area, went to their local VM shop, and they did it there and then, took about 10 mins or so.. Appears that some have a 'can do' attitude, whilst others have a 'can't be bothered' one. (might be to do with a commission based salary?? If you are not selling something, (and an existing sim size swap isn't a sale) then there's no incentive to do it, or spend time that you could be selling something to another customer perhaps??)
I phoned my local store and they said they'd do it. I went down there and even after turning down getting the phone from their on a contract they still upgraded my SIM, and they upgraded my contract to give me a bit more data for the same price. So really very good service, especially when considering they didn't make any more money off of me.
I'm not sure if I should mention the store or not if this is the thing that they shouldn't be doing, but I'm very impressed with them.
I'm not quite so impressed with the person on the main phone line that I spoke to this morning who told me that this couldn't be done...