Yes, if you upgrade you can choose which version you want as long as you are paying for the upgrade. What is this plotter though? I really don't recommend downgrading to 32 bit, unless it is necessary for your system to run properly. But if 64 bit was installed at the time you bought your computer, I don't see there being any conflict with the base system. In very rare cases does a manufacture install a OS that doesn't run properly or effectively on the machine in use.
If this plotter is a software you are having troubles with, try looking on the net for a patch or fix for it to run correctly on Windows 7 64 bit. I myself haven't ran into any issues with software, but games yes.
If it is hardware, do the same. Look for the correct drivers online.
Downgrading should really only be done if there is absolutely no other option. Pc's that are capable of running 64 bit OS's, tend to run better, faster, and smoother when using it. Using 32 bit on a 64 bit system is fine, but it doesn't allow for the hardware to be used to it's fullest. Thus, your computer is not running to it's fullest capabilities and hardware is really just being wasted. Kind of like buying a PS3, just to play PS2 games only on it, doesn't make any since.
