So I own a Giant Boulder and currently changed its tires to some slicks and it is way different from its mountain bikes tires, the slicks are thinner, way lighter and faster. So I noticed I can go faster and wanted to know if it is possible to change the cassette to have more gears. I currently have 7 gears on the back and wanted to put maybe 8 or 9. It is possible to change the cassette or is just for road bikes. Also my shifter only goes to 7 so I need to change it also right? Also the derailleur?