Are you wanting to switch from Japanese SP to English XP?
If so, yes, you need to get the full version (not the upgrade).
By the way, I'd recommend doing some research first. Many people have had an EXTREMELY hard time finding drivers that'll work for their hardware in an English OS.
Does the company that makes your laptop also sell an identical English version model? If so, then you shouldn't have problems finding the drivers.
To be completely honest, I am not sure WHY it is such a problem (I have never gone through the process myself). Drivers themselves are universal and are not language dependant. It is the INSTALL programs that change, but you theoretically SHOULD be able to point windows (when it asks for the drivers) to the driver files and install them. SHOULD.
But there have been a few otherwise completely literate computer people who say they have not been able to do it. Urban myth perhaps, but thought I'd warn you in advance.