From what I can understand, if you have an HSDPA/EDGE card, no matter what signal you are picking up, you will pay the higher rate (premium).
You can (from 5 June) swap back to a refurbed 3G card, and a migration needs to take place on their billing system (which will only take place on the 1st day of the following month), and then you will be billed at the 20% lower price of 3G/GPRS. You will, however, be limited to 384kbps on 3G.
You can do these swapovers only once every 30 days. So if your tower is not 3G enabled, or edge enabled, and you only get GPRS, it makes sense to stick with the old 3G datacard. Then once the tower is covered, do your swap and migration should you want the higher speed of HSDPA/Edge.