It can change the sustain, resonance, apparent frequency center of the guitar, harmonic content, and evenness (dead spots or lack of). Everything, in other words. Sometimes a little, sometimes a lot. You might not realise how much if you've never changed a set, but it really can be dramatic sometimes. Contrary to popular belief, light heads are not always better. Nor are heavy ones. Not even necessarily if you know what kind of tone you're after - it's the way the mass of the heads affects the resonance of the neck that's important, so it's difficult to make any kind of generalisation that applies to more than one specific type of guitar (or maybe even one specific guitar). It even depends on what type and gauge of strings you use. That's why I would never advise irreversibly modifying a guitar to take different heads unless there is no alternative - it might improve things, make no difference, or make them worse. And one person's 'improve' might be another's 'worse' anyway... That's the big thing I have against Sperzels - they simply don't fit without modification in most cases.