changing 7 speed to 8?

G'day all - I have an older XC bike that I'm setting up with slicks as a commuter. Its currently running Shimano LX gear in a 7 speed. It's getting pretty tired and old and I was wondering what my best course of action would be? Understandably I don't want to spend a lot but do want to get it up and running nicely. Ideally if I could get hold of some good condition 7 speed shifters, derailers etc that would do me but what would be involved in switching over to 8 speed?
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