O2 contract question

Hi,

I am getting an iPhone 3G S next friday, and I was wondering something... the £35 plan is all I really need as 600 minutes is more than enough for me, but I read somewhere that after 9 months you could downgrade your contract with O2.

So I could start with the £44/month contract thus saving £100 upfront (have to pay £175 instead of £274), and after 9 months downgrade to £35. This way the "total amount" of money over the 18 months would be the same (around £890).

Thanks!