[inquisitor]

I don't know what exactly happened when you swapped Blackberries, but I'm sure the phone itself is tied to the BIS-subscription by it's IMEI (serial number).
Where did you get your new Blackberry from? If you have purchased it from T-Mobile they could have registered it automatically. Otherwise they perhaps register new and so far unregistered Blackberries automatically if the SIM card belongs to an account with active BIS subscription.

Regarding your second question I don't have an answer. But what I certainly know is, that France is probably the most expensive country in the Western World when it comes to mobile internet access. I would be surprised if any operator offered BIS for prepaid subscribers.

What about plan C, going to Italy instead of France? There you could get prepaid BIS at reasonable rates and they won't try making you eat snails.