We use high-resolution K-band VLT/HAWK-I imaging over 0.25deg^2^ to study the structural evolution of massive early-type galaxies since z~2. Mass-selected samples, complete down to log(M/M_{sun}_)~10.7 such that "typical" (L*) galaxies are included at all redshifts, are drawn from pre-existing photometric redshift surveys. We then separate the samples into different redshift slices and classify them as late- or early-type galaxies on the basis of their specific star formation rate. Axis-ratio measurements for the ~400 early-type galaxies in the redshift range 0.6<z<1.8 are accurate to 0.1 or better. The projected axis-ratio distributions are then compared with lower redshift samples. We find strong evidence for evolution of the population properties: early-type galaxies at z>1 are, on average, flatter than at z<1 and the median projected axis ratio at a fixed mass decreases with redshift. However, we also find that at all epochs z<~2, the most massive early-type galaxies (log(M/M_{sun}_)>11.3) are the roundest, with a pronounced lack of galaxies that are flat in projection. Merging is a plausible mechanism that can explain both results: at all epochs, merging is required for early-type galaxies to grow beyond log(M/M_{sun}_)~11.3, and all early types over time gradually and partially lose their disk-like characteristics.