  The ground-state spins and magnetic moments of $^{49,51}$K have been measured
using bunched-beam high-resolution collinear laser spectroscopy at ISOLDE-CERN.
For $^{49}$K a ground-state spin $I = 1/2$ was firmly established. The observed
hyperfine structure of $^{51}$K requires a spin $I > 1/2$ and from its magnetic
moment $\mu(^{51}\text{K})= +0.5129(22)\, \mu_N$ a spin/parity $I^\pi=3/2^+$
with a dominant $\pi 1d_{3/2}^{-1}$ hole configuration was deduced. This
establishes for the first time the re-inversion of the single-particle levels
and illustrates the prominent role of the residual monopole interaction for
single-particle levels and shell evolution.
