A 7-speed freehub body is too narrow to accept an 8,9 or 10-speed cassette all of which fit the same freehub body. So you would need either a new rear wheel or a replacement freehub body. Also, your frame's rear dropouts are most likely spaced 126 mm while 8-speed and above are 130 mm so you would either have to "cold set" (read bend) the dropouts wider or struggle with every wheel removal and installation.