No, it should be quite doable, but you probably have to send a stop-signal to NSSM for the service - then change the setting in the mountscript and restart the service. It's not hard at all, but I guess it's slightly less convenient than just CTRL+C, hitting the uparrow and chaging one letter on the line.
I'm doing a little cleanup on a simple mount-script template now so it is a bit more presentable for public consumption. A few small robustness things and some more appropriate commenting.