Note to all people that are planning to port their numbers over to new N900 contract but within the same network (ie. I have T-Mobile number and got another new T-Mobile contract with N900 and I'd like to port my number onto that contract).

Turns out that because of the way how PAC codes work, after porting number to 'gap' network, it will need to remain there for another 30 days, until new PAC code could be issued, and that unfortunately applies to all networks in the UK. :(

I just ported my T-M number out to Vodafone pay-as-you-go service and then lady explained that 30-day PAC code rule. I then called T-Mobile again and they actually confirmed the same. Having said that, unfortunately I'm now stuck with pay-as-you-go service for another month. A bummer!