We report measurements of ac magnetic susceptibility Xac and de Haas-van Alphen (dHvA) oscillations in KFe2As2 under high pressure up to 24.7 kbar. The pressure dependence of the superconducting transition temperature Tc changes from negative to positive across Pc ~ 18 kbar, as previously reported. The ratio of the upper critical field to Tc, i.e., Bc2/Tc, is enhanced above Pc, and the shape of the Xac versus field curves qualitatively changes across Pc, dHvA oscillations smoothly evolve across Pc, indicating no drastic change in the Fermi surface up to 24.7 kbar. Three dimensionality increases with pressure, while effective masses show decreasing trends. We suggest a crossover from a nodal to a full-gap s wave as a possible explanation. [ABSTRACT FROM AUTHOR]