I'm hoping someone perhaps familiar with phones and T-Mobile's offerings might be of assistance.
After reading through this and similar threads, I'm about to buy an N900. I currently have the National 400 plan from T-Mobile (an older plan, I believe) on a Sony-Ericsson TM506.

With my current phone, can I change my plan as directed in this thread and switch the SIM with the N900, or do I need a more advanced handset than the TM506?