Apple has posted a Knowledge Base article on swapping SIM cards in iPhones, which the company says may be required for "repair, warranty service, or if you own an authorized unlocked phone (Germany and France only) and want to use a different network provider's SIM."
The gist is that SIM cards can be swapped, but if they are, the user should reconnect the iPhone to iTunes subsequently in order to ensure proper operation. Apple says "If you do not connect iPhone to iTunes after changing or reinserting the SIM, iPhone may not work as expected."
The article makes no mention of swapping SIMS when upgrading from, for instance, an 8GB iPhone to a 16GB iPhone. Technically this should be possible, but some users have experienced difficulty with the process, instead needing to re-activate the 16GB phone using the new SIM card.