Accordingly, how much does it cost to change a pickguard?
Do you have the pickguard already or are you looking for that to be included in the price? A real nice Greven will run about $35 USD. The bench charge might be another $30 USD or so depending on where you are. Hope this helps.
Also, is it hard to change a pickguard? Replacing your pickguard is a relatively simple undertaking, but as with any guitar mod, it's important to plan ahead by having the right tools and understanding each step of the operation.
Keeping this in view, are pickguards necessary?
Pickguards are used to protect the finish of your guitar. If you play with a pick and want to preserve the finish of the guitar then a pickguard is necessary. If you don't use a pick and/or don't mind minor scratches or signs of wear from strumming a pickguard isn't strictly necessary.
Does a pickguard affect tone?
Pickguards have an incredibly small affect on the the tone of the guitar, so small in fact, that you'll probably never be able to hear it. The impact of pickguards on tone is even less prevalent with electric guitars because the tone wood doesn't matter as much as it does with acoustic guitars.