If it is a LM4558 opamp or such it won't care. I would make sure that any electrolytic caps are rated at more than 9 volts. If they are 6v caps they might pop. I can't see that using it as a preamp will hurt anything. It might just sound like crap. The input and output loads may not be matched for a guitar and amp. An easy way to bring the power supply voltage down without a regulator if you need to is to put rectifier diodes in series on the B+ side. Each one will bring it down .6v or so. 4 foreward biased in series would bring your 9v closer to 6v. Or you could just replace the caps. BTW, there are some really good guitar preamp kits out there that do the same thing right that don't cost much. General Guitar Gadgets comes to mind but there are lots of others. They have schematics on line though which might be usefull in modding what you have.