Dammit. This is good overall but it means i'm going to have to get a new contract if I get a new iPhone.
Unless mobile carriers somehow come up with a way for you to transfer sim data.
Since a new contract won't actually transfer any SIM data at all, I assume you mean associating the SIM with your account.
This is something every carrier can do within seconds. They just remove the current SIM and add the new one to your account. So far as the network is concerned, it's just associating SIM 124 with the account instead of SIM 123.
So you can just walk into any carrier shop and go 'I'd like a new SIM please, my old one doesn't fit.' and they should set you up if they have nano-sims in stock.