[inquisitor]

@stu
The UM data SIM and the ICQ SIM are not the same! They differ in roaming countries, pricing and especially in the session-setup-fee.
Whereas ICQ customers pay only € 0,39 for each day of data usage, UM data customers are charged € 0,39 for each datasession. And since datasessions are regularly interrupted (e.g. if you're out of coverage, if handovers fail or if a cell is overloaded) you must reckon with extra costs for the session-setup-fees of the UM data SIM.
Anyway as +423 customer you'll only be able to switch to the UM data tariff and not to the ICQ tariff. Also keep in mind, that voice tariffs of the UM data SIM are worse than the +423 SIM in many places - so you should exactly compare the prices for the countries you regularly travel to before you switch.