You can switch cards at will. Sidekick and G1 plans are for specific devices, but TMo is moving away from that model. Be aware that before, the cheaper data plan was non3G. I'm not sure about now, but I had been told a year ago that that plan was for non smartphones with 3G. They can tell if you have one of those devices because there aren't many, and they're branded, so probably have some sort of user agent to know which device it is.

To make things easy, just spend the extra $5 and cya. The service is phenomenal in my area.