Yes - at least you can with Three. I did that last year whilst trying to decide whether to upgrade or change network. The one I was on was a rolling 1 month contract which meant I wasn't tied down once I made a decision.
When my iPhone 4S contract ended I went onto sim only with virgin - £15 per month with unlimited everything including data! My bills are always less than £20 a month and hardly ever over the £15 really. I'm using the money I save each month to put towards buying a new iPhone outright and keep the sim only because its such a good deal! Might be worth considering sim only until the new iPhones come out?
I have a 4S and contract is up so went to upgrade but my head is spinning with all the options!
I want a 5AM for sure but do I stay with O2 or move to EE? The coverage is much the same. Or do I wait for the expected new Iphone release at the end of the summer so just go into a rolling contract for lower amount?