If it's the same model etc. and the drivers have already been loaded then it should be a simple enought task to swap out.
However, there can be some issues of logistics if it's a differnet model that may require you to :-
1) Remove the exisiting drivers
2) Remove the card
3) Reboot the machine without a modem inserted
4) Install the new drivers
5) Install the new modem
The above is basically some of the experience I have had in the past.
Unfortunately, until you receive the replacement, you won't know what needs to be done. Hopefully they will include full instructions.
The physical swapping of the modem cards is a simple process. Do you have an anti-static wristband?